Me and My girlfriend tryed to lose our virginities earlier, but I couldn't get erect. Why?

I'm really confused as to why I couldn't become erect, I am VERY attracted to my girlfriend, and i've never had this problem before. I'm not sure if it was nerves, but I don't think so because we are both really open and okay with each other. I obviously felt humiliated/ashamed/embarrassed, but she was great about it, and we ended up talking in bed for another 2 hours. Any help? As I would obviously prefer this not to happen again.