Oops...I think I just pissed off my boss...Can you help?

One of my supervisors just called me up to see if I could work overtime on a project I was supposed to be helping her with all afternoon, only she's just now done with her end of it and it's 30 minutes to closing time. Normally I would stay and help, but I have plans tonight and I can't stay. I told her I'd come in early tomorrow to help but she said it has to be finished by tonight. Then I made the mistake of saying, ';Well, I've been ready to help you all afternoon, and you weren't finished yet.'; So now she's mad. What do you think I should do? I don't want to get fired...