I clicked on a screen resolution in OS X that was incompatible with my monitor and/or monitor cable. I can restart with the shift key held down and boot into a usable resolution, but if I restart in "normal" mode, my display is useless. How can I recover from this?
After booting with Shift held down, go back into System Preferences and choose a known good resolution—that'll write it back to the PRAM.
Alternatively, you can just wipe the PRAM by starting up holding Command-Option-P-R until you hear TWO startup chimes.