What makes a graphics card not work?

Yeah, I've been wondering how they actually shut down and not work anymore.

I've had 3 different graphics cards and all have ended the same.

1st was my Geforce 5200 - this was with my for like 2 years. I understand that this might have been a wear and tear issue.

2nd was my Geforce 7600 - this may have been because I replaced it for the 5200 which was using an old system... dunno.

3rd is my recent Geforce 9800 - I have had my new comp for 2 months and this is the thing that decides to die on me.
I was in the middle of reformatting my 1TB HDD because I had a virus I couldn't get rid of.

Do you guys know what things would make the graphics card shut down?

There's the issue of overusing the computer but there are people that go more hardcore than what I do on the comptuer and their comps seem to have longer lives.

Spikes or dips in power. A PSU that is struggling to keep enough wattage going to it. However, the number one reason for Video card death is heat. These things can take all kinds of heat but once you starting passing 90C, it gets ugly. To keep the temps down make sure the case has good airflow and clean the card out with compressed air. That's my .02. ;)

Would a 550W PSU struggle with the 9800GT?

no you could even put a 295GTX ;-/

Physical damage from moving the case or something, overheating from bad ventilation (not enough fans) or over-usage, and not enough power as people have said. I think some of the circuit components actually have life-spans, maybe they are just getting really old... My friend had a card that stopped working because part of it melted (so he tells me).Sammojo

If all your comps keep dying maybe it's the power in your house? Try buying a surge protector with a line filter on it to clean up the power.

Which brand is your PSU?

overheat from a bad cooler, a bad overclock or from dust in the cooler