Hmmm? Did you try wiggling the power button, or depressing it on center and then side to side? I have a Palm TX that does that, and I have press it a couple different ways to get the Unit to turn on and off. I have since found a software resolve for that problem.
I've never had that problem with my X51v, but perhaps others know of a similar software resolve like I found for my Palm. By searching PocketGear, I have found the following that might help:
http://www.pocketgear.com/products_search.asp