Hey my name is Joe and I need some help. See I really like this girl. Heres the situation

See this girl and I were pretty good friends at the begining of this school year, nothing romantic. We have two classes together, Biology and a Weight Lifting PE class. Anyway as the year went on we started to talk a lot and hang out at school together a lot. It was really easy for me to talk to her and I would even play games with her like thumb wrestling and stupid little games like that. We had a lot of fun. I even asked her to go bowling with me. See Im kinda a shy guy but this was the first girl that I felt really comfortable with. I really started to like her. Ever since I really started to like her though it has gotten really difficult for me to talk to her. I get really mad at myself when I cant think of anything to say to her. I really would like to talk and hang out with her again like I used too. I would really appreciate it if somebody could help me with my situation. Any help would make me very very happy.