Grey veil, what is it?

[Severin Crisp]

A strange one!  For no obvious reason a grey veil dropped down across 
the screen which was fully visible but muted.  The computer froze and a 
message box, white text on black, in English, German and Japanese said 
that I needed to restart.  A press of the restart button gave a normal 
restart and all seems fine.
This did not look in the least like an OSX message and at first I 
suspected a virus or similar.  Is it perhaps from Unix?
Information would be most welcome!
Severin Crisp