iMac can't detect its own display

Hi

I've been troubleshooting a GPU/display issue - weird bands/stripes and boot problems, which are now resolved - sort of. Now I just have a black screen, but the imac is booting OK and I can screen share/file share from my MBP. I can even facetime using the built in isight! But when I go to system info in 'about this mac', and click on the display tab it tells me that no display is detected.

I had thought this was a problem with the video card - that is what all the googling and threads on here suggested. But I can see the screen properly while screen sharing, which suggests to me that the video card is doing something right, but the images it is rendering are not making it to the display. Or am I fundamentally misunderstanding what the video card does?! Either way, does this now sound like a display issue, rather than a GPU issue? If so, am I looking at a new LCD (for whcih read, new imac)?

Thanks

Colbec

Specs: Intel iMac 2008, 2.66 core 2 duo, 20 inch, ATI Radeon HD 2600 XT, OS X 10.9.2