Mac halts after boot and restarts

Hi,

My mac 2012 had black screen and fan yesterday. Then I powered off it and retried. This time it has loaded Catalina desktop and a few seconds later restarted.

Now when I ry to power on either it shows black screen with fan sound or it desktop, restarts.

I did NVRAM PRAM reset.

I tried pressing D on keyboard to diagnostics but it did not load diagnostics.

I tried recovery mode. It entered recovery mode but when in language selection halted and restarted.

I plan to unmount, then try unplug and plug some cables inside.

I appreciate any idea or suggestion.

Thank you very much.