Won't boot beond Apple Logo and spining wheel

My iMac wont boot up past the white apple screen with the spinning circle. This happened after my mac froze then i had to turn it off by holding down the power button. Any help would be wonderful and also what this means. Unfortunately i lost my start up disc so this probably doesn't help.

thanks very much.

How to Clear the PMU 1. Step 1 Quit all of your active applications and shut down your Mac. 2. Step 2 Once your computer is completely shut down, disconnect the computer's AC power. 3. Step 3 Press and hold down the power button on your iMac for at least five seconds and then release the power button. 4. Step 4 With your Mac still shut off reconnect the AC power. The PMU/SMU has now been reset on your computer.

Next try resetting the PRAM. Start up and immediately hold down the option - command and the "P" & "R" keys. Wait till you hear three tones, then let it try to start up

I've done all of the above, still have apple logo and spinning wheel.

I got a restore disk, but it was the wrong rev. I think the machine told what the correct was. But I can't get back to that point. How do you the OS from the CD drive

Hardware checks ok using internal LEDs