Saturday morning my pc suddenly shutdown. It went off like the power cord had been pulled. In a fraction of a second, as it was going, I though I saw a pop-up bottom right of the screen.
It took me four attempts to get the pc started. A few minutes later it went again. The shutdown was so dramatic I started thinking of hardware issues such as the graphics card failing - especially as a few reboots suggested Windows was running but with a blank screen.
This happened a number of times the same: the pc would just shut off and take numerous attempts to boot afterwards.
I finally saw it on Sunday - The pop-up was Bitdefender Wallet. A bit of reaserch told me that even if you have it switched off (as I did) it still has programs in the background and these can cause the sudden shutdowns I'd been having.
I disabled pwdmanui.exe, pmbxag.exe and bdapppassmgr.exe in startup programs and the problem is solved.
It may not happen to you, but it's something to be aware of.