Cougar,
I believe it to be the White screen ransomware virus as it appeared on a couple of occasions which lead to a reboot and then it became apparent that it was something nasty when the screen went white shortly after log on and the drive was then locked on reboot.I assume there was a countdown until lockout.
Machine is 3 years old based on an Asus P8Z77-vlx MoBo which is UEFI
Loading OS via genuine W7 64bit DVD then MS W10 download.
Just sorting a bootable Ubuntu usb.
Both running PC’s scanned with MWB last night and no issues,PHEW!