I would go further into the suggestion of inviting her out, along with several other friends. Go out for ice cream or a slice of pizza. You don't need to wow her, but if you want to be her friend, make her an offer she can't refuse.

Another point someone mentioned is that maybe you are being a downer. I've lost some friends being a downer myself. I know it is not intentional, but sometimes the face gives it away. Practice smiling in the mirror. Seriously. When I go to work or before an interview, I'll spend the fifteen minutes before the start time to simply smile in my rear view mirror. Smiling is just as important at work as it in with your friends and family.