If the computer's running Mac OS X 10.6.8 or earlier, insert a compatible Mac OS X install DVD and restart with the C key held down.
If the computer's running Mac OS X 10.7 or newer, press the Command and R keys.
Use the Disk Utility to repair the internal drive. If there aren't any problems, install an OS onto it.
It won't let me boot to a disk, In fact I have the Install disk in the drive and it won't even let me eject it. Its not taking any keyboard commands at boot.
Five ways to eject a stuck CD or DVD from the optical drive
Ejecting the stuck disc can usually be done in one of the following ways:
1. Restart the computer and after the chime press and hold down the
left mouse button until the disc ejects.
2. Press the Eject button on your keyboard.
3. Click on the Eject button in the menubar.
4. Press COMMAND-E.
5. If none of the above work try this: Open the Terminal application in
your Utilities folder. At the prompt enter or paste the following:
If this fails then try this:
Boot the computer into Single-user Mode. At the prompt enter the same command as used above. To restart the computer enter "reboot" at the prompt without quotes.
My iMac won't boot into singe user mode, it won't eject the disk and does not respond to any keyboard entry or mouse entry. After a couple of minutes, it just goes to the white screen with a file folder and a question mark. I just replaced the HDD about a yearago; becuase that failed, so the HDD is realtively new.