You're gonna have to borrow another HDMI monitor or computer for diagnosis.
The most likely culprits are:
HDMI cable failing. Common with cheaper HDMI cables as the contacts corrode over time.
Monitor going bad. Was common when there was a spate of faulty Chinese knock-off capacitors installed in tons of computer equipment by nearly all manufacturers. I have seen many monitors fail slowly recently as these capacitors are finally blowing up.
I've only rarely seen a video card in a computer go bad. They do, but not as often as the two above.