iMac won't boot

First, tell us more about your iMac.

What year was it made?
Which one is it?
Which version of the OS were you running?
Can you boot while holding down the shift key?

I know I sound like a broken record, but problems like this illustrate why one should ALWAYS keep an external fully-bootable cloned copy of the internal drive nearby.

You want to have "a second boot source" close-at-hand if you get into an "I can't boot!" moment, just as you're having now...

 

Sounds as if the hard drive went south without you!

 

Do you still have the original OS X recovery DVDs that came with your iMac?
If not, you might try booting to the hardware test (restart, holding the D on your keyboard)

With Snow Leopard, you won't have a recovery partition on the hard drive.
But, you could try booting to Internet Recovery (Restart, holding Option-Command-R)
You can run Disk Utility from there, to test your hard drive - if it even shows up in Disk Utility.
And, Internet recovery will also offer Lion to reinstall, if your hard drive is working.

 

Tim wrote:
"Hardware test reveals no problems but cannot boot. All suggestions are welcome."

Sounds like a corrupted OS.
Then again, could be a hardware problem with the internal boot drive (but wouldn't the hardware test pick up on this?).

Can you boot to recovery mode?
If so, try an OS re-install.

Otherwise, you need a BOOTABLE EXTERNAL DRIVE of some sort.
Something that can "get you to the finder".
Or a bootable USB flashdrive with an OS installer on it.

I always have a second fully-bootable drive online and available to me, ALWAYS.
It makes problems like this far FAR easier to diagnose.

 

Do you mean that safe mode doesn't make a difference - or that you can't boot into safe mode at all?
Your posts all lead into the likelihood that your hard drive is failing, or failed already. As safe mode uses the same boot partition, then you can probably expect that safe mode won't work either.

The hardware test does not do any extensive test of the hard drive, and really will pass the drive, as long as the drive responds as a storage device (but doesn't test to see if it is actually bootable) Your Disk Utility is a good test for that, and the fact that you can't boot to the drive is really the best test.
Did you find your original installer DVD?
Have you tried booting to Internet Recovery? Again, restart, holding Option-Command-R. If successful, you should see a spinning world (not the Apple icon). That will let you run Disk Utility to test the hard drive directory, and you can also download and install OS X. The internet recovery will upgrade you to Lion.

 

Tim wrote that...
... can't boot to safe mode
... can't get to recovery partition, either?

Again...
... You're going to have to come up with "an external way" of booting.

It could be a USB flashdrive that has a copy of the installer on it.
It could be a DVD copy of the install software
It could be an external, bootable drive that is capable of booting your computer.

Do you live within distance of a brick-n-mortar Apple store?
If so, I suggest you make an appt. at the Genius Bar and see if they can help with obtaining something that could boot the Mac...

Otherwise, you're going to keep "goin' in circles" and getting nowhere...