Chime, black screen, no PRAM reset, help!

Hi folks,

I've had my 2308 iMac since new. It was working just fine, and then one day I came to switch it on and got the chime, but nothing else. Blank screen, no response, etc.

I have attempted to reset the PRAM/SMC with wireless and wired keyboards, and tried to boot in Recovery mode: nothing.

I also hooked up an external display: again, nothing.

Having looked though the vents on the bottom, I can see two diagnostic LEDs lit (as this model has integrated graphics, I'm assuming there should be three lit?)

Any ideas?

If it helps, service history: about 4 years ago, I replaced the original Seagate drive (broken). Everything seemed to work fine after that, although the optical drive stopped reading discs.