Need help investigating startup error

Hello guys, I have a problem. My father in law's pc has a problem. I am tech savvy myself, but I am not sure what happens here. The PC powers on and shuts down after a sec. I checked the front IO connectors, everything fine. The Pc doesn't even get into the BIOS. It worked fine yesterday, but it wasn't started for two years before. I haven't built this PC myself, he bought it at a Shop like 4 or 5 years ago.

It has an old Ryzen, a HDD for Storage, an older graphics card that is just there for the display out, some No Name RAM, a corsair Power supply (500w). All of the connections seem fine.

What could cause the PC restarting constantly without even getting into the BIOS? HDD propably not, ofc the Mainboard itself could be dead, but is there another solution that could cause this?

Looking forward to your answers and thank you for helping!