iMac on a reboot loop

I've tried some of the other threads relating to this subject but none seem to apply to what I'm experiencing. I have a 2011 27" iMac that quit suddenly and is now on a reboot loop. It gets to the point where the grey screen with the apple icon and progress bar appears but once the bar gets to around 1/3 of the way, it shuts down again and attempts to reboot. I eventually needed to power it off with the power button. I've tried leaving it unplugged with the external HD disconnected as well, but there was no change to the problem. I have not done any mods to it aside from maxing out the RAM, but that was a few years ago and it has been running fine until this issue popped up.

I realize it could very well be the hard-drive giving up the ghost but thought there might be something I'm missing. Any ideas on what that might be or if there are any work-arounds for this?

Thanks very much.