iMac Won't Power Up / Start Up

I was trying to restore an iPod Classic and sync but it froze the iMac (I think perhaps the iMac had too many processes happening at once as there were lots of applications open). I did a Force Quit and now the iMac doesn't want to start. I have tried all methods I can find. Holding down different keys etc (resetting SMU / PRAM) and also leaving the iMac off overnight but still nothing. The machine powers on but then it sounds like it goes to sleep about 5 seconds later (you hear a click like something cuts out) but the power is still there as you can hear it if you listen to the back. No display / no chime / won't boot from installation disk (which is still inside iMac).

To make matters worse I think part of the problem is that the iMac is not recognising the keyboard. I have tried both wireless and USB but neither ones shift lock lights go on when the iMac tries to power up. This probably means that SMU / PRAM have not been reset at all in my previous attempts.

Any suggestions?
iMac, Mac OS X (10.7.2)

If its not booting at all I would first try and re=seat the ram, then go from there

Thanks - I think this is the one thing I haven't been able to do. Did attempt it but they did not want to come out easily - it says pull tabs but even when pulling forceably then did not want to budge. I've an appointment at the Genius Bar tomorrow so was hoping they would do that as I don't want to break it.