I suffer this problem occasionally and could not work out why. I have started to track this down recently to when Windows 10 or Microsoft is doing a silent install (which it does too frequently when not active).
When the computer wakes up from power save it wants a Password or PIN but the correct one is not accepted. If you use START:POWER you see that the computer asks for "update and shutdown" or "update and restart". I pick "update and shutdown" and then boot the computer; it now accepts the normal password or PIN to logon as usual.
There is something strange when these silent installs happen in Windows 10 and the computer needs restarting because nothing tells you this after waking up from power save and your computer Password or PIN will not be accepted. This is a really annoying feature in Windows 10