gdgt

You may want to try resetting the PRAM on your iMac. The PRAM is a special bit of memory that stores things like display settings and volume settings, and clearing it out will sometimes fix strange issues like this. Simply shut down the computer and turn it back on while holding down Command + Option + P + R. Keep holding those down until the computer reboots again and you hear the startup chime play a second time.

(If you're having trouble getting that to work and you're using a Bluetooth wireless keyboard, only start holding down the keys when the startup chime plays. It may not have connected to the keyboard before that point.)

If that doesn't work, you could also try resetting the SMC, which controls various functions related to power, sleep mode, and the ambient light sensor that adjusts display brightness. To do this, shut down the machine and unplug it, wait 15 seconds, plug it back in, wait 5 more seconds, then turn it on.