Windows 7: Uhmmm...Gray Screen (Of ....)... What the hell does a gray screen mean

I opened Sacred while waiting for a backup finish... and a gray screen covered my screen with the mouse busy icon. It doesn't accept neither to close the program or wait for the program to respond.
Can ctrl+alt+del and choose start task manager but it turns back to gray screen.
No problem, a restart will solve the problem (doing a backup with three external devices attached so have to wait till their busy signals stop to avoid future problems).

I was just wondering what a gray screen means. I hope not something bad. Thanks.
(Please don't tell me my graphics card is failing).

EDIT: Even choosing log off at the ctrl+alt+del section solved the problem.