Can anyone help me get my iMac to reboot? I get a gray "curtain" over the start up screen saying I need to restart my computer but it won't restart. I just keep getting the same message every time.

Try to boot in safe mode. The instructions provided by Apple are as follows:

• Be sure your Mac is shut down.
• Press the power button.
• Immediately after you hear the startup tone, hold the Shift key. The Shift key should be held as soon as possible after the startup tone, but not before the tone.
• Release the Shift key when you see the gray Apple icon and the progress indicator (looks like a spinning gear).

If you can boot in safe mode, reboot as usual (i.e., not in safe mode.)

That's a kernel panic, a fairly serious failure that usually (but not always is caused by a hardware problem).

See this article, and try the fixes in it:

http://support.apple.com/kb/TS2570

Also, try what is suggested in this article:

http://support.apple.com/kb/TS3742

What is your Mac OS version?

Are you hearing the start up chime?

I had a problem like this quite some time ago and it turned out to be badly seated RAM memory. I powered off, unplugged the machine, removed then reseated the memory. Problem sorted (for me at least).

Check your RAM memory and remove then reinstall it.

If the problem continues, you should contact Apple Technical Support for further options.

Hope you get it sorted.

Yeah, as necronym stated, it is a hardware issue and more than likely a RAM problem. Disconnect everything from the computer except keyboard and mouse and try again. If it still happens try removing one and/or swapping your RAM sticks over. Keep trying until you have eliminated all possible combinations. Just put a mark on one RAM stick so you know what have tried. If it still fails to boot properly you should probably replace your RAM, but that is no guarantee of success. Have a look at this link for more info on a Kernel Panic.

The X-Lab - http://www.thexlab.com/faqs/kernelpanics.html

Does the display look like this?