This is the first thing that I'd try.
1) You should run disk utility from your system cd/dvd.
a) verify the disk
b) update your permissions.
2) Try a safe boot.
Shutdown your machine. Hold down the shift key. Poweron. Wait awhile Wait awhile while you harddrive
is being checked.
Sometimes if volumes don't appear in Startup Manager (what you get when you hold down the Option key at startup), you need to reset the Mac's PRAM, NVRAM, and Open Firmware. Shut down the Mac, then power it up, and before the screen lights up, quickly hold down the Command, Option, P, and R keys, until the Mac has chimed twice more after the powerup chime. Then, before the screen lights up, hold down Command-Option-O-F until the Open Firmware screen appears. Then enter these lines, pressing Return after each one:
"The reset-all command should restart your Mac. If so, you have successfully reset the Open Firmware settings."
How to eject a cd from the internal cd drive:
List of devices:
List of variables:
You may want to reset the PMU (power management). How you do that depends on the specific iMac model. It generally involves disconnecting the Mac from power, with some additional steps. Can you identify your iMac model precisely? This web page has a list of all iMac models, to help you.