First of all, trash and refresh your Photoshop preferences.
To re-create the preferences files for Photoshop, start the application while holding down Command+Option+Shift (Mac OS). Then, click Yes to the message, "Delete the Adobe Photoshop Settings file?"
Note: If this process doesn't work for you while you're using a wireless (Bluetooth) keyboard, attach a wired keyboard and retry.
Important: If you re-create the preferences by deleting the Adobe Photoshop CS6 Settings file, make sure that you only delete that file. If you delete the entire settings folder, you also delete any unsaved actions or presets.
Reinstalling Photoshop does not remove the preferences file. Before reinstalling Photoshop, re-create your preferences.
NEW Video! Julieanne Kost created a video that takes you through two ways of resetting your Photoshop preferences. The manual preference file removal method is between 0:00 - 5:05. The keyboard shortcut method is between 5:05 - 8:18. The video is located here.
Mac OS
Important: Apple made the user library folder hidden by default with the release of Mac OS X 10.7. If you require access to files in the hidden library folder to perform Adobe-related troubleshooting, see How to access hidden user library files.