If you have spent much time in the Gmail Help Forum you've probably seen long lists posted by people of things to try. Unfortunately some users will skip these list: "My problem is much too complex to be fixed by clearing the browser cookies." Or they will skip some steps thinking: "There no way any of the Gmail labs could cause this." But much too often that is incorrect and they fail to fix their problem by skipping something simple they could have tried.
So it's important to follow some check-list like the one below anytime you have a problem in your Gmail account. With luck, one of the items will fix the issue. If not, you may be able to narrow the problem down to something much more specific making it easier to identify and fix. And if they don't help you will have at least ruled out the most common causes of problems.
Of course there are some things these obviously won't help with. Clearing browser cookies won't help you remember a forgotten password. If messages you sent to another account bounce with an error, trying it on another computer probably won't help. And disabling browser extensions is unlikely to help you recover an accidentally deleted message from Trash. On the other hand, none of these will make things worse so it never hurts to try them if you're unsure of the cause of some problem.
So before you post to the help forum, ask someone for help, or pay for third-party support, make sure you've tried all these simple fixes first. You should also write down the results of each one and be prepared to share which items (if any) helped with the problem and which didn't.
The following list is basically in priority order. That is the first items have the best chance of fixing or isolating the problem. Some items, like clearing the browser cache/cookies, may fix the problem. Other items, like trying another supported browser, are diagnostic giving you more information about the nature of the problem. Be careful about skipping any as some side-effects can be subtle and totally unexpected, like a lab causing the Help choice to be missing from the gear-icon menu (Help link gone missing in Gmail).
Clearing the browser's cache and cookies, then exit and re-starting the browser.This simple item tends to fix more strange problems in browsers than anything else. Many people make this a normal part of their computer usage doing it on a regular basis. And it's always the first thing to try if there's something unusual going on with Gmail.
This article provides instructions for several major browsers: http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=32050
Updated your browser to the latest version.There are many problems that can be caused by running an older, no longer supported version of your browser. It is also very important to be using a supported browser with Gmail. This not only means which browser you are using (Firefox, Chrome, IE, etc) but that you have the latest version. If your browser is unsupported or out-of-date, fix that next.
Supported Browsers: http://mail.google.com/support/bin/answer.py?hl=en&answer=6557
Try using another supported browser and another computer.Many problems are caused by using an old version of a browser that is no longer supported, and often browser will simply act differently. Testing your issue on several browsers will help you determine if it's browser dependent (only fails on one browser).
Trying a different computer can help you rule out some computer-specific causes. And you should always test a computer on a different network (like at work if you normally access Gmail at home) to rule out any network/firewall/ISP issues.
This step is diagnostic. Discovering that your problem only happens in one browser doesn't fix it, it just suggests that it's a browser issue instead of something else.
Disable Internet Explorer's compatibility mode.If you use Internet Explorer, you need to disable the compatibility mode which defaults to enabled: http://mail.google.com/support/bin/answer.py?hl=en&answer=181472. The fix will not work unless you also un-check the 3 options at the bottom: "Include Updated Website lists from Microsoft," "Display intranet sites in Compatibility view" and "Display all websites in Compatibility view." After making the changes restart the browser.
This is clearly a browser-specific fix and is strongly indicated if the previous test showed the problem only present in IE.
Try both the Standard version (https://mail.google.com/) and Basic-html version (http://mail.google.com/mail?ui=html) of Gmail.This is another diagnostic test which can help rule out account problems. If you are logged into the Standard version of Gmail you can test the Basic version by simply opening its URL in another browser tab. This makes it easy to do a side-by-side comparison of the problem in each version.
Disable all browser extensions and add-ons.Browser extensions can cause all sorts of unexpected side-effects. This is especially true of any extensions specifically designed to interact with Gmail. One should always try disabling everything that's been added to the browser to see if the problem goes away. If it does, a little additional testing can usually determine which specific extension/add-on caused the problem.
Related to this test would be testing in an incognito window (Chrome) or running Safe Mode (Firefox).
Disable/delete any browser toolbars (like Ask, Bing, Yahoo, etc).While similar to the above test involving extensions/add-ons, this one specifically addresses disabling or deleting any toolbar programs you have installed for your browser (they may not show up as an extension/add-on). Again, you simply want to rule these programs out as a cause.
You can typically uninstall these by going to the Control Panel \ Programs and Features and looking through the list for any toolbars you have seen displayed in your browser.
Disable any labs features you are using.There are many useful functions available in Settings->Labs. But as the disclaimer at the top of that page says: "They may change, break or disappear at any time." It is important to rule them out as a cause of a problem by disabling them all and re-testing. You can disable all of them at once by using: https://mail.google.com/mail/?labs=0
Temporarily disable your anti-virus scanner.Some problems in Gmail, especially related to missing parts of the user interface, can be caused by an anti-virus program blocking access. But it’s critical that you also disable any internet or e-mail related extensions/add-ons it has as the blocking may be specific to internet activity or e-mail.
Don't forget to re-enable it after the test.
Disable any other monitoring or internet protection programs (like Net-Nanny).There are other types of malware, ad-blocking, and child protection suites one may have installed. Like the anti-virus test above, these should be temporarily disabled to see if the Gmail problem is being caused by their activity.
Uninstall and re-install your browser.This is related to the above test making sure your browser is up-to-date. But sometimes if a problem seems to only happen in one browser, it's worth re-installing it on your computer to clear any problems with the installation that may have developed. If you don't tell the uninstaller to clear settings, all your personal information should still be present after the browser is re-installed.
Supported Browsers (contains links to download various browsers): http://mail.google.com/support/bin/answer.py?hl=en&answer=6557
Disable Settings->Offline->Offline Mail if you use it.It's pretty rare for Gmail Offline to cause a problem, but if nothing else is helping it's worth testing. See: Settings->Offline
Verify that your computer's system date, time and time-zone are set correctly.There are some problems with web-site security certificates as well as e-mail time-stamps that are easily fixed by making sure the time settings are correct on your computer. Do not overlook the timezone setting. You can typically access this by clicking on the time displayed on the task bar, and selecting "Change date and time settings...".
Delete and re-installing Flash and Java on your computer.This probably isn't one you'd do unless there was some indication that there was a specific problem with one of these packages. But again, it never hurts to make sure all the software on your computer is up-to-date and cleanly installed.
Oracle Java: http://www.java.com/en/download/index.jsp
Adobe Flash: http://get.adobe.com/flashplayer/