Give her the benefit of the doubt. As someone above suggested, maybe she was in the hospital or maybe some other emergency came up and she wasn't able to contact you or forgot. Whatever the case, assume it was beyond her control and attempt to reschedule.
You have no legal grounds for charging a no show fee as there was no contract, verbally or otherwise binding her to her appointment.
If you do persue this with anger and an attempt to recoup monies for whatever reason, you can count on her never having her vehicle serviced with you - ever! And, she will do everything in her power to tell everyone she knows and comes in contact with not to use your services.
It's a lost-lose situation.
Being nice and biting the bullet will most likely help you retain the customer but if the reason she didn't make the appointment was something like I mentioned above and she see how understanding you are and still are willing to do her vehicle, she'll be your best customer with regard to referrals.
Sometimes we have to take the high road whether we want to or not. It's just good business practice.