External monitor suddenly changed resolution and won't change back

I've been using an external 1680x1050 resolution Acer 22" monitor with my 12" Powerbook for the past year without any trouble. Last night when I connected to it, the screen resolution was suddenly set to 640x480 instead of the usual 1680x1050 and the displays were mirrored. So, I un-mirrored them and hit "Detect Displays" and the LCD monitor resolution changed to 1024 x 768. Worse, the correct option of 1680x1050 isn't even available! I tried resetting the PRAM and turning everything on and off but it's still messed up. What the heck happened? I'm a designer and can't work on a screen that's all stretched out! Help!