Is your side bar now stuck at the bottom of the page? Are you using WordPress? Did you recently make some layout changes to your site? If that’s sounding (and looking familiar) it’s a problem I just squished on one of my sites. In essence its likely you have a mismatching <div> </div> pair in your code. For every opening <div> you should have a corresponding </div> at the end.
I recently made the mistake of editing some code and chopping off an opening <div>. The result propelled my sidebar to the bottom of random posts, while launching the WordPress comment section into the sidebar. I spent forever flushing caches, trying different browsers, disabling and enabling plugins – it was none of those. It was a simple case of missing div.
If you’re seeing any sudden randomised layout issues using WordPress that you just can’t explain, retrace any recent steps in editing your code! If you can’t spot the issue yourself, have a Google for “HTML validator”. There are a number of free tools out there that will help you pinpoint any errors in your code.