Perhaps she could be shy? Or as PDM said, she may not have received your message.
Does she know who you are?
You could start off with just chatting to her about a recent movie or band and asking what type she likes. Get chatting first and then ask her out for a coffe i.e. 'can I buy you a coffee or drink'. Make it simple.
If she says no or that she's not interested then accept it because at least then you'll know the answer and who's to say she won't contact you again after thinking about it. If she's the shy type she may just say no because of that.
