I have a Audi A6 Quattro 2002 and it was running just fine a week ago I un hooked the battery for 20 min and then I hooked it back up now the car won't start.

i had the same problem and my car was diagnosed with a bad timing belt i decided to sell the car these audi can be a very big expense good luck

You likely wiped out the cars computer memory. Doing this can cause the key to some times become un-sinked with the car. it can also misalign the throttle body. try doing this .. Put your key into the ignition and turn it to power on (do not turn it over) leave the car for 15 -20 minutes like this. When the time is up flip the car to power off then try turning it over. This has a 50% chance of actually working. If this does not work get the car scanned with rosstech and have the throttle body realigned.