there is a switch on the top of the transmission that over time looses is water proofing and is prone to getting wet (at least on the A6 from 97) if this is the same problem the Audi dealership should be able to replace and repair the problem for less than $100. I had the same problem a year or so ago on my A6 i think it cost me $75 to have it fixed. But there is also the chance that the puddle had nothing to do with it and you need tranny fluid the no reverse s the first symptom of a leaking tranny that needs fluid. But most likely it is the switch that got wet and is now bad.