Same thing happened on my old Asuspro - new SSD and Windows 10 addeda few years ago. No amount of hard booting would get rid of the blue password box????
Took the battery off - noticed ‘Lock’ slider wasn't over, thought nothing of it. Powered on and there's my Windows desktop exactly as it was, even browser windows open!!
Replaced battery, locked it on ? and everything's good again. No I have no idea either ? just happy I don't need a new hard drive.