Windows won't accept PIN after power outage, doesn't show an option to put in password

(This is a complicated one, as I'm supporting my parents over the phone with this)

My parents' home computer shut down due to a power outage. Now, when they turn it on, the login screen says that they've put in the wrong PIN and asks them to type in A1B2C3 - enter image description here

Once they do that, it asks them for their PIN again, and when putting that in (they're certain it's the right one), and following that it says This sign-in option is disabled because of failed sign-in attempts or repeated shutdowns. Use a different sign-in option, or keep your device powered on for at least 2 hours and then try again.

Yet, there is no other sign-in option. Specifically, there is no way to enter a password.

Any idea how we could get them to use their computer again?

More information and things we tried

There is only one admin account on the computer, so they can't log in with a different account (I'll remedy that as soon as we're back in)
I tried to do a System Restore (by restarting the computer whilst holding shift), but it asked for a password and rejected any password we put in. I've reset my father's password through Microsoft's website, but it seems like the computer hasn't fetched the new password.
Tried to log in in safe mode, but the same process with PIN repeated itself.
When on the log-in screen, the computer is connected to the internet (through WiFi)

Any idea what we can do? Seems like the PIN is borked and I'm looking for a way to log in with a password (preferably one that will fetch the new password from Microsoft)

Thanks!