Why can't I force my xoom to restart?

my Xoom stopped working. Will not restart. I've tried forcing restart via Start button and Volume keys, nothing happens. When plugged into power source I some times get a green light other times a white light.

I can hear alert messages but still only see a black screen. When connected to computer via USB I can see data such as pictures and music which are functional. So not sure whats going on. What are your thoughts?