I lost my gift from my father.

I don't know how u can ask him to return the computer to u through the law.
However,Ｉthink u can ask him where or who he sold the computer to? Then, u should buy the computer back directly from that person.
Actually, I think u can report this matter to your father honestly. I think your father can understand and will forgive u. I think your father will tell you so now u've learnt a lesson from this experence - don't lend any valuable thing to anyone unless u've known that person very well and many years. To make it simple, just buy another new one and forget this friend forever.
Actually, there are some people very bad. I helped a colleague to record one TV program with my own tape. After she finished watching the tape, she never returned the tape to me. The tape is not expensive, but from this small matter, u can know what kind of person this colleauge is. However, I have another colleauge - she asked me to borrow a hair string to hole her hair since hers was just broken. Then, she returned the hair string to me the next day.
So, sometime u must learn some lessons from experience.