Q: Shaded screen with message to force shutdown by holding power button down

Hello, I have an Intle Icore 5 mac.  It is running on OS 10.7, on several occassions I get a shaded gray scren that come from the top of the display screen.  It then show a message box in several languages telling me to shutdown by holding the power button down until the computer shuts down.  What is this, and why is it happening.  Not sure if there is a fix, or if it is normal.  Thank you for any help!