27 inch imac died after RAM uprgade

Hi Mac gurus,

Just had a bad experience upgrading RAM on a late 2012 27" iMac. I attempted to add 2 x 8 RAM to the empty RAM slots - should have been a simple procedure.

After I added the RAM modules (PC12800 DDR3 1600mhz So-DIMM 204 PIN RAM), the iMac booted with error tones. I after a few attempts at resolving the issue, I removed the new RAM and reverted to the original RAM. The iMac booted with the usual chime with the circular progress icon and then immediately shut down with a black screen.

It was suggested I reset the PRAM - I did this following instructions on the apple support site and was presented with the disc utility. I ran the verify disc and the program said to run a repair. I did this and rebooted. Now it won't boot at all, and won't even allow a PRAM reset. It just cycles through the boot chime - goes black and repeats.

So now I have a dead iMac. Anyone with any suggestions? I'd really like some advice.

Thanks.