How to reset Photoshop preferences (In Windows):
First, hold down all three Ctrl + Alt + Shift buttons. Now "while keeping those button held," simply open Photoshop or a file that opens with Photoshop, and as Photoshop loads, you should get a prompt asking if you would like to "delete the Photoshop settings file", Click yes
You should now have a fresh slate with all of the settings returned to their default.
On a Mac, hold Shift + Option + Command while starting Photoshop to delete the preferences.