Omg what do I do?

Hello
You guys help me out and the past with advice but I need help again.
As some of you know recently me and my gf broke up but the reason is suprising.
See she thinks that I smoke weed and thats the reason she broke up with me.
The problem is I want to be with her but she won’t believe me and someone told her about this lie.
What should I do to make this right?
She won’t even talk to me and I at least want to be friends.