I have a Dell Latitude 5500 Laptop with Debian 10 and KDE Plasma 5.14.5. I've had this laptop for about three months, and this just started a few days ago.
When I'm done for the day, I usually press ctrl-alt-L to lock it, then close the laptop lid. When I open the laptop lid the next day, both the external USB keyboard and the built-in keyboard do not respond. The only way I can unlock it is to use the mouse to go back to the switch session screen, and then both keyboards (external and built-in) will let me type.
I think this has something to do with closing the lid because if I lock it but leave the lid open, I've never had this problem.
I went almost three months without having this problem.