Did my video card die?

My XFX Radeon 4770 has always been sensitive to dust and begins to idle at around 50°C after 6 months of applying new paste and cleaning the fan. Well, yesterday I came home and I noticed it was idling around 70°C with the auto-fan turned up mighty high. I cleaned up the card, new paste and all and now my computer won't even POST. The NumLock turns on and then the keyboard is unresponsive, monitor never powers on, and harddrives aren't loading Windows.

I've tried swapping PCIE bays, different PSU power cables. Nothing. It will always boot with onboard video, but ONLY if I've taken the 4770 out completely. I'm looking to get a friend's video card to see if it'll boot from it. Before I rule out that my loyal, 6 year old card is dead could I have missed something? Could it be that I seated the heatsink wrong, etc?

Thanks in advance.

I would just quickly and closely inspect the card for any damage or burn marks in case something burned out so you don't risk further damage.

Nothing that I can find now or smelt anything burning when I came home. I've smelt burning silicon before and that is a smell that is hard to forget (as a neglectful 11 year old, I once chucked in a DDR backwards. THAT was memorable! ) That isn't to say something didn't get fried, as I was away and came home to a then-functioning but hot card.

Was there a red led/diode on the back of the pcb?

This is what happened to mine ,the fan on my sapphire 4770 started to make some noise, showed high fan speed but it would be wobbling. It overheated while gaming and rebooted. When the pc restarted there was a red light on the pcb. After it cooled down, i restarted and it was alright minus the fan was still dying. Strapped on a 120 mm fan with tie wraps and it's even better.

good luck

No red led... When I turn on the PC, the fans turns on and everything is as it should be until 3 seconds have elapsed and still no display and no feedback from the keyboard. I've tried reseating the heatsink and fans to no avail.