About 10 years ago I had a similar problem..

I managed to identify the fault and solution.

The PC needed to be unplugged, and then the graphics card needed to be pushed firmly into it's slot.

Reconected to the power supply,reboot, then it started working again as normal.

Apparently, over time the card had vibrated slightly adrift
from making good connection with the motherboard slot contacts.

Also blasting with an air gun [my giotto rubber air bulb] to remove any dust from the contacts.

This happened every few months afterwards, requiring the same solution.