my Imac doesn't reboot: screen is all white

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.

http://support.apple.com/kb/ht1455

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:

reset-nvram

set-defaults

reset-all

"The reset-all command should restart your Mac. If so, you have successfully reset the Open Firmware settings."

http://support.apple.com/kb/TS1812?viewlocale=en_US

How to eject a cd from the internal cd drive:

eject cd

List of devices:

devalias

List of variables:

printenv

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.

http://www.everymac.com/systems/apple/imac/index-imac.html