Below is a LIVING list of helpful troubleshooting tips to fixing Sitecore issues.  Meaning, these are tips that Sitecore developers typically will run through before going to the Sitecore Community and/or Sitecore Support.  If a tip is not in this list, feel free to comment and I will add it to this running list.

These are tips, in no order, that might be helpful to aid you in determining the root of an issue and coming to resolution:

• Check the Sitecore Logs for errors located in the Data/Logs folder.  It’s a best practice to have the Data folder as sibling to the Website folder and not inside the Website folder.
• Using Chrome’s Web Developer tool you can check the Network tab, or in Firefox Firebug use the Profile button to see if anything is failing with when being loaded or events are firing etc.
• See if the issue is browser specific by using different browsers (IE, Firefox, Chrome).
• Debug with Visual Studio for ASP.NET/C# issues.
• Debug with Firebug/Chrome Web Developer tool for JavaScript type of issues.
• If .dll issue take a look at the .dll in .NET Reflector, or similar reflection tool, and see what the code is setup to do.  This might lead you in the direction of the issue.
• Try clearing the cache at http://[yoursite]/sitecore/admin/cache.aspx
• Check to make sure your includes are being included post .config build at http://[yoursite]/sitecore/admin/showconfig.aspx
• Try recycling the app pool for the website.
• If missing files/items see if a previous package was installed and reinstall to pick up the files/items.
• Is the issue environmental, meaning works in one Sitecore instance locally, but not in staging/production? If so, what could be the difference in files/items/configuration you can compare that will give you direction to solve the problem. You may need to use a tool like Beyond Compare, or similar tool, to diff some files to get to the root of the issue.

Thanks in advance for any helpful tips to help out the Sitecore community! Happy coding!