1) First, go to international student office, ask for the copy of your visa. They must have it. As I remember my sisiter's visa has been copied by the lady named Heater in registration office.
2) Check www.cic.gc.ca and Call CIC hotline, they must have solution for your visa.
3) Becuase you don't have evidence, you can do nothing even you go to see the police. Be careful next time.