Grey screen crash?

[Selwyn Ward]

Just suffered my first crash in OSX.  At least that's what I guess it was.
I'm using my 17"Al when suddenly what looks like a pull-down shade runs down
the screen from top to bottom, making entire screen contents look grey
tinted. A notice appears (carrying the power button logo) announcing in
multiple languages that the computer needs to be restarted.  No buttons work
except the power/restart buttons.  Anyone else encountered this?