What has probably happened is that your preferences file may have become corrupted. In fact, I had the EXACT same problem and resetting my preferences file was the advice I found in the Adobe Knowledge Base. It worked!
Luckily it's quite simple to fix this. As quickly as you can after starting Photoshop (don't wait for the the splash screen), press and hold <Alt><Control><Shift>, if you are on a windows system. On a Mac press and hold <Option><Command><Shift>. A dialogue window will appear asking to to confirm that you want to delete the current settings.
This is just one of many odd things that can happen when your preferences file becomes corrupt. Whenever PS starts behaving oddly I consider creating a new preferences file (as referred to above) and 99% of the time that's all it takes.
Tip: When you have reset your file...take your time and make sure that you have all your preferred settings in place once again and then IMMEDIATELY save a backup of the preferences file to another location on your hard drive so that you can restore it easily the next time your file becomes corrupt.
The location of the preferences files vary by PS version, but if you go to Adobe's site and search the term "Location of Photoshop preferences files" you'll find a document that tells you where to find them!
Hope this helps!