iMac No Boot, Some Display

Hi all,

I'm wondering if you can help. I've done some Googling, but can't find any mention of this specific problem.

I've got an early 2009 Intel Aluminium iMac, and it's not booting at all. When you hit the power button, it chimes, the screen flashes grey, and then goes black. There's no Apple logo or progress bar.

I've replaced the memory to no avail, and reset the NVRAM/SMC.

Any ideas? Any help is appreciated.

Many thanks,

Gavin