There is a failure in SUSE update I have seen on 2SLED and 1 SLES.
The actual kernel update conflict with Nvidia driver.
The screen stay gray.
What to do:
Ctrl+Alt+F1 get to a screen.
Login as root
search for Nvidia
update all drivers, but mark them from bottom up - not up - down.
After update of driver close Yast
now the screen is back. The problem is solved.