Failure to pay for services

Christa hired me on a once off basis to cover a BMW event for her for Thu 26 Feb & Fri 27 Feb 2015. Her client failed to manage their time on Thu night for me to go home and rest, to be able to prepare for the next day. I texted her at midnight notifying her that I wouldn't take the chance of driving back the following morning after two hours of sleep following a 20 hour day that I had at the event on Thu. She is therefore withholding payment for my 20 hours of work for Thursday 26 Feb 2015 just because I did not report for duty on Fri 27 Feb 2015, after I expressley notified her that due to poor time management of her client on the first day, I wouldn't manage to cover the following day.