Why won't this girl give me my envelope back?

i gave this girl an envelop ewith a card inside a while back because i wanted to get with her. well i got tired of her and found out she was talking with another guy, so i asked for it back. but she said she didn't have it, so she doesn't want to give it back. why won't she give it back? she is a shy girl, and i am an attractive guy, so it gets me even more confused