When a friend or family member comes to you asking for help with a program that's not working correctly, the first thing most of us ask is: "Did you try restarting?" When I'm talking websites, it changes to: "Did you Shift+Refresh?"
Note: For the purposes of this discussion, I'm assuming the person with the problem still has an active internet connection and every other website is working just fine.
Sure you could clear your entire browser cache (in Firefox: Tools -> Clear Recent History), restart your browser, or go nuts and restart your computer, but most of the time that's overkill. Any of those methods could be worth trying eventually, but before you try anything more drastic or time-consuming, just hold your Shift key and click the Refresh button in your browser (or press Ctrl+Shift+R [Win] /Cmd+Shift+R [Mac] ). That clears the cache for the files in that page so that you're sure to be running the latest and greatest version of what that web site's serving.
If That Still Doesn't Work?
A cache-overriding reload isn't a fix-all, but it is the first thing I tell anyone to try ("Hold Shift and click the Refresh button") when they're having a problem with a website. If it doesn't fix the problems you're having with a website, you've still got plenty of troubleshooting paths to exhaust (clear your entire cache, restart the browser, restart your computer, uninstall extensions, etc), but it's a good place to get started.
Have a method you swear by as your first resort when the websites you rely on go a little wonky? Let's hear it in the comments.