Why does my G4 go comatose periodically when in sleep mode?

[Ronlou]

G4, X-10.3.9. When I try to "reawaken" my computer when its been in sleep mode for a while, the screen stays black. I've tried moving the mouse, using key strokes, pressing the shut down button on the computer or on the tower, and nothing works. I need to restart the computer... Any ideas, anyone...?