2001 Audi A6 cranks but won't start sometimes
 

Hi- My son's A6 2.8 with 91,000 miles was running great. The other day it would crank and crank but never start? He went out the next morning and it started right up? Hmmm...He took it to school, it drove fine and after school, it would not start again, it would only crank. It will not start at all now. The car has been sitting at the school lot for 2 days. Thankfully we live next to the school. We tried to start it 3 different times a day for 2 days and it will not start at all anymore. I do not have the means to test codes and may have to have it towed but I was looking for advice on what it may be? Especially since it was fine, then would start, then was fine for week, then wouldn't start, then started the next day, and now won't start at all...Puzzled.

Any theories?