iMac loading bar gets stuck halfway then reboots
Have you recently changed anything in the hardware?

I had this happen once trying to swap drives between two Macbook Pros. They were different hardware versions (a couple of years apart), and the hard drive from the older Macbook did exactly this when trying to boot in the newer Macbook - the old version of the O/S didn't understand the newer hardware, and would get to a certain point, and then reboot.

After I put the drive back in the original (older) Macbook, I upgraded to the latest version of OS X, and was then able to put the drive into the newer Macbook without issue.

This doesn't sound like what you're doing (you didn't mentioned swapping any hardware), it does sound like you're having an issue with hardware that can't be properly started. So I'd be assuming hardware issue at this point, which hopefully someone can assist you in how to diagnose that.