Lion hangs on boot with empty progress bar and then switches off
One more thing to try:  Boot in Safe mode holding the Shift key at the startup sound.  If the internal drive is bad (failed drive or borked OS), this won't work.

If you have an external drive as a full bootable backup (and I hope you do), connect it a power it up. Then try booting the Mac holding the Option key, and select the external drive.

If you have another Mac, connect the sick Mac to it and boot the sick Mac in Target mode (hold the "t" at startup sound).

If either works, copy all critical data to the drive you are booted in.

I agree that there is at least one hardware failure involved.