HP laptop keyboard not recognized on wakeup; fixed by restart - why?

My HP ProBook 450 G5 (winver 1903, build 18362.657) wouldn't recognize its built-in keyboard when I accessed it this morning. A simple restart of the computer fixed the problem, but I'm wondering why it happened.

Some background:

I always restart the computer at the end of the day (the workday; it's my office computer). Usually when I open it - the next morning, or the Monday after a weekend - the login screen immediately appears. If I'm gone for more than a two-day weekend - as I was this time - I have to press the power key to wake the computer up. But however I get to the login screen, I get there and log in.

This morning - again, I had to wake the computer with the power key - I tapped key to bring up the signon part, and nothing happened. Same with multiple taps. Clicked the mouse and the password field appeared, but then I couldn't type anything. It's like the built-in keyboard wasn't there.

Did a simple restart using the mouse, and when the login screen appeared, everything was normal.

I'm not at all worried about the problem reappearing. Just bothered and curious, about why this happened at all.

I would first suspect the BIOS (CMOS Memory). Reset the CMOS memory, with the PC powered off, power charger disconnected and PC battery removed (if its a removal battery). This requires opening the back of the laptop to access the CMOS memory battery (usually a CR2032 coin cell battery). Remove the battery for at least 10 minutes then reinstall. Considering the low price of a 2032 it may be just as well to replace it while your at it. Put it back together and see what happens on a "cold" start.

To add: The CMOS memory clear/reset is one of those things that can fix many flakey problems. Reason I suggested it.