Going off Scott's comment, I'm going to write out how to reset preferences (based on the video) just in case someone just wants to read over this and do it rather than watching the 8 minute video.
Method 1: Moving the Preferences Files
First off, make sure you are closed out of Photoshop.
Next, if you're on Mac, navigate to:
~/Users/[Username]/Library/Preferences/Adobe Photoshop CSx Settings
Or On Windows 7:
\Users\[Username]\AppData\Roaming\Adobe\Adobe Photoshop CSx\Adobe Photoshop CSx Settings
Next, move both
Adobe Photoshop CSx Prefs.psp and
Adobe Photoshop x64 CSxPrefs.psp to somewhere else (i.e the desktop).
Now simply start Photoshop and your preferences should be reset.
Method 2: Keyboard shortcut
Hold down the following keys:
Mac: Command + Option + Shift
Windows: CTRL + ALT + Shift
And then launch Photoshop, a dialog will come up asking you to confirm whether to delete the preferences or not.
Please note that the keyboard shortcut method is a little more destructive and will delete some other preferences that Method 1 will not get rid of. So method 1 is recommended unless there are no settings/preferences you'd like to hold on to.