My freshman year a friend in a class called me up to ask me when the final was. The final started at 11 and went until 2 PM. I finished in a little over an hour and was home when she called at 12:30. I told her that the final was going on at that moment. She didn't believe me for a couple of minutes, then she took off for school. She got to the final a little before 1 and the prof let her take the test until it was over at 2PM.
So, yeah, thats my story. Get ahold of your prof and be completely honest. The worst thing that could happen is that you would eat the bad grade. Hopefully if you are honest your prof will allow you to makeup the exam.
Ryan