Any blinking lights ? As the system wakes up, per @CWOTUS‘s descriptions. A lot of motherboards have replaced the beeps with lights or a light that blinks in diagnostic sequences.
Can you determine what kind of motherboard you have? If so you might be able to get some diagnostic info from lights.
Besides the graphics card as @Thammuz has suggested possible, it can be RAM.
If you haven’t reseated the RAM chips try that. Also, assuming you have multiple RAM chips, you can try taking them all out and replacing only one at a time.
Some motherboards require sets of two mem chips be installed at all times, but most do not any more. If you can get ahold of motherboard documentation you could find that there is a slot #0 that you would put a single chip into, and #1 the next etc. although again a lot of motherboards don’t care what slot the RAM is in.
I had a shoebox Shuttle freeze up completely when one of the mem slots on the motherboard stopped working but with the other slot and whatever RAM chip it worked just fine. (Replaced the mobo last summer). Worth a try.
Do you have an onboard graphics adapter on the motherboard? That would at least let you see the boot sequence. There would be a separate video out connector on the back of your machine if there you have a graphics card and an onboard. Pull out your regular graphics card completely and hook the monitor up to the onboard connector and try a boot.
Good luck!