OK, there is this girl who had this massive crush on me, I liked her but I didn't want the hassle of having a girlfriend then.

However, now the tables have turned and she's into this other guy who she is seeing an now I really like her an don't know what to do!? Can someone please give me some helpful advise?

It done, you didn't want girlfriend and she moved on, unless you can flirt with her and win her back, or she is just doing this to make you mad, just ask her to be your girlfriend

I would start talking to her more, if you don't have her number ask for it. Call her and ask her to hang out, test the waters see if she still likes you at all, and if she will come back to liking you. Once you know then make the next move. If she does then you would obviously ask her out, if she doesn't I would try to move on and let it drop, you can't make a person like you.

Good Luck