How do I ask this girl to prom?

So there's this girl I like. I've known her for about two years and went to Homecoming Dance with her once. Thing is, I want to go to prom with her this year. However, I'm not sure if she likes me. I hung out with her last week at an anime convention for the first time in a loong time. We got reaquaninted and all that jazz. Today, I asked my friend to tell her I still have feelings for her. She just said I don't know, maybe. My friend then proceeded to ask her what would happen if I asked her to prom. She said "He's late." She has a date, but it's a girl. I guess they are going as friends since I don't think she is biual and such. That's the thing, I don't know whether I should ask her to prom or not. I wanted to make a card and decorate it asking Will you go to prom with me? I just wanted to know if you guys think I should go through with it still? I might wanna take a chance on this one seeing she has liked me in the past before. She might reconsider going with a real date instead of another girl. So should I go through with it? Or forget about it? Sorry for the long thread, I just want to know some opinions.