Zapping the PRAM should reset your display preferences to default. Restart your computer while holding down these four keys on the keyboard: Command (apple key), option, p, and r. If you do it right, you will hear the startup chime, a long pause with no activity, then the startup chime again.
After that, you can let go of the keys, and let it boot as normal. Good Luck!
(us paranoid techs usually let it chime 2 or 3 times for good measure, but i think it's just superstition)