Try to get the good out of this...
If you feel you have disappointed her, find (not make up) a plausible reason why you were unable to attend, and apologize graciously. Ask her if there were pictures made, and if another event is planned. She'll be OK, and you don't have to beat yourself up!
It is probably best, I still believe, to consider her previous responses to be a very polite, minimally insulting, "no." She may have declined, but she didn't brutally shoot you down, so she may feel kindly toward you. If she has a heart at all, she'll be flattered at your interest, and, if she has any class at all, she'll treat you well socially from now on.
This does not necessarily mean that you "have a chance" with her. These things can be beyond your control and beyond hers as well. But if this enormous outpouring of support for you on this forum means anything it means that folks here find you interesting, charming, funny, and popular! We can't all be wrong (except Mad Jew)!
Only you can decide if all the fashion and presentation advice applies to your case. Like I said, if you call yourself a "nerd," it might! Everybody everywhere will tell you things like, "if you apply for a job, dress like you already work there." I would argue that the same is true for your hot date -- you need to dress and hold yourself like you belong with her before she, and her friends, will consider that you could.
Like I said, too -- you're a great guy and have great prospects beyond this girl. If you finally lose out on her, you just have to shake it off and carry on. College is better!
PS: You need to do a little self-examination and figure out why you actually didn't go to the show. It sounds like your confidence just failed you -- but you can't squander your bold opening moves by dilly-dallying later!