Physical damage is pretty obvious, just take a look and see if anything is broken. Often see screw driver marks on cards that are installed with little care and attention. Improper storage of loose parts. But if it has just been sitting in the chassis, and you didn't do anything too rough with it while installing your fan, probably not it.
How much tension did you apply to the zipties or whatever attachment method you used. Too much stress on the PCB could bend it slightly and make it lose connection with a few pins in the slot. Try re-seating the card in the slot.
Stress tests and benchmarks are the best testing methods for an end user. If it crashes, artifacts, or becomes generally unstable, then you have your answer.
Could also be your monitor that glitched out for a second, bad connection or something else. I would just wait and see if it happens again, if it doesn't, hooray. If it does, start testing things.
Swap out the monitor cable, try a different port on the GPU, swap out the monitor (if possible).
Capacitors can go bad over time, and random failure in any component is possible.
Really you should be investing in a new GPU in the near future anyway.