Why does the PC screen go dark after entering the PIN?

My mother has a laptop with Windows 10.

When starting it must enter a 4-digit PIN. That was what my late father had set up.

Has also worked the whole time, for a few days it is like this:

She enters the PIN and the screen goes dark and nothing works.

How could I fix this?

Maybe numbers reversed? Was the PIN written down somewhere? Otherwise, it is also possible to reset passwords with a special password reset CD.

My mother is 83, my dad used a distinctive PIN.

So… That's not the problem. But still…