From the Photoshop help files:

Restore all preferences to default settings

Do one of the following:
Press and hold Alt+Control+Shift (Windows) or Option+Command+Shift (Mac OS) as you start Photoshop. You are prompted to delete the current settings.
(Mac OS only) Open the Preferences folder in the Library folder, and drag the Adobe Photoshop CS Settings folder to the Trash.
New Preferences files are created the next time you start Photoshop.