Audi A8 start problems
 

I have just changed the altenator on my 1999 a8 quatro and after getting done with the change out i started the car and it ran for 2-4 mins before I turned it off I let it set for about 24 hrs before I needed it agian and I went out to start it agian and it turned over and didn't start the battery was up it sounded like it was missing spark or something.
The car has never missed a start so this is really out of the ordinary for this car.

Can anyone tell me what to look for or what may have happen?