Well, you could actually open the back, but it would take a lot of effort and may void any warranty. If it is under warranty, I would really suggest contacting your provider or the manufacturer before doing anything with the phone on your own. Now if it is not under warranty then you can look into fixing it yourself, if possible.
You say he took it off the charger and I am assuming it was working when he did, but when you got it back it wasn't? If that is the case, what did he actually do with the phone while he had it? That will help determine what the problem is.
Also, you can try holding down the power button for about 20 seconds and see if it starts.
Last question, when you plug it in now, does it show as charging? Or does...