Has your copy of Photoshop started to behave strangely? Some things work - maybe most things work - but some things that used to work, don't any more? The first time this happens to you, it can be extremely puzzling because so many functions do work fine, except the one thing you you want to do do then and there. What may have happened is that your preferences file may have become corrupted. 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.

The one problem with this is that there are rather a lot of settings - eight different sets, as you can see: 

So it's a good idea to make a note of any that you have changed from the default values before you do this.