Well you only have a few options.
1. Go to his house and ask for it back nicely. If he says no, you can either react nicely and ask to have a conversation with him or react violently and aggressively. (Or you could do this over the phone of course.)
2. Go to his house and ask for it back aggressively.
3. Go to his house but with your mother, ask her to help you through it.
4. Call the police (but I don't think you want to do that).
5. And lastly, there is of course the option to leave it and get a new computer later.
Personally, I would suggest 1 with the nice attitude. Good luck though, I feel for you man. :c