Upstairs, downstairs

I couldn't get it up and I'm 19! It was going to be the first time for me and my girlfriend, but the man downstairs didn't respond! I didn't feel nervous, I felt confident that I would be fine. But when the mood struck I couldn't deliver. Now not only do I feel like crud, I'm hoping that the next time it happens that the little soldier downstairs decides to take some action... It felt like the most disappointing thing in my life. I was tired. If that is a factor, then maybe I just need a good nights sleep and another go at it? Any advice would be amazing.