This could be anything from a fried capacitor to a virus. You indicate having ruled out a bad power supply. I assume you can not get to a recovery partition.
You might as well see if you can get the machine running, just to narrow down, possibly rule out hardware malfunction. Here is the download site for creation of a bootable USB flashdrive. If you can use the flash drive to get the computer running, perhaps the problem is not a bad motherboard.
https://pcsupport.lenovo.com/us/en/lenovodiagnosticsolutions/tools/bootablediags
If a virus has taken you down, you might try the Norton Bootable recovery tool. Yes, it goes on another USB stick. If your computer will run (not a bad motherboard), this software should boot it and scan it for a virus.
https://us.norton.com/support/tools/nbrt.html
Here is a generic page about non booting computer rescue. I thinki you have a computer running XP. You might check dust accumulation inside it.
Here What to Do When Your Computer Shuts Off During Startup
Once you eliminate a physical cause and a virus, see if you can run chkdsk, boot to safe mode, or access the recovery partition. At some point, you may have to try TestDis, Easeus, or Paragon utilities.