Try this:

by mrmacfixit Moderator - 10/29/09 5:21 PM

In Reply to: iMac Start Up Problems - Gray Screen by TrollChat

Remove the power cord from the iMac and leave it out for 20 minutes.
Plug it back in and power it up again.

It is not unusual for a malfunctioning internal drive to hold the boot process up, or even stop it altogether.

Do you have any form of warranty on it?

P