Resetting your Photoshop preferences can fix problems that you are having within Photoshop, and is a good thing to try. Remember, that resetting these preferences will put all of them back to factory default, so you may need to readjust some things after you do this.

There are two different methods of resetting your preferences. 

Keystroke reset

Mac

Holding down Command, Option + Shift while starting up Photoshop will cause a dialog box to appear. You will be prompted to delete your Photoshop settings file. 

Windows

Holding down Alt Ctrl + Shift while starting up Photoshop will cause a dialog box to appear. You will be prompted to delete your Photoshop settings file. 

Choosing "Yes" will delete the settings file that Photoshop creates and updates whenever you make a change in your settings. 

Manual reset

This option will provides a targeted approach.

Mac

• Macintosh HD/Users/[Username]/Library/Preferences/Adobe Photoshop [Version] Settings/Adobe Photoshop Prefs.psp
• Macintosh HD/Users/[Username]/Library/Preferences/Adobe Photoshop[Version] Settings/PluginCache.psp

On OS X 10.7 and later, you can access the user library displayed below by clicking on Finder > Go > Hold down the option key > Library. Holding down the option key will display Library in the Go list. It is hidden by default.  

Windows

• C:\Users\[Username]\AppData\Roaming\Adobe/Adobe Photoshop [Version] \Adobe Photoshop [Version] Settings\<any.psp file>

The 2 files that we remove most commonly is the "Adobe Photoshop CSx Prefs.psp" and the "PluginCache.psp"

The Adobe Photoshop file is going to reset your preferences for Photoshop as a whole.

The PluginCache file forces Photoshop to reload all of the plugins that it has knowledge of. This has the possibility of uncovering problems that you didn't know you had. Deleting the file did not cause the problems.  The newly created version of this file found the existing problems and reported them.

Both of these file are re-created by Photoshop (if deleted) the next time Photoshop launches.