Do you think this girl liked me? + Why would she stop talking to me all the sudden?

Yes she did not only like you, but she found you physically attractive as well. Her sitting on your lap indicated she was sexually attracted, and if she kept wanting to talk to you...then she liked you. Maybe she found you a bit young, IDK. But you sure should have asked her out before it was too late. Well what i would do is say"hi(her name here)! havent talked to you in a while, what's going on?" then yea.. carry on from there and try to talk to her again before you start hitting on her.