Why is your gerbil not moving?

This may be a cause of illness. If your gerbil shows lack of interest in food and water, then it may indeed be sick. Does it spend a lot of time sleeping?

Also, is your gerbil alone? Gerbils need companionship because if they are alone too log, most gerbils actually die of loneliness. They lose interest in a lot of things they used to do when they had their companions. My advice, go to a vet and get him checked out.