Dead battery. I accidentally discharged my battery last year, overnight, from a Friday to Saturday. Everything you described was the same for me. But I realized it was my fault, I left the door ajar, I was using VAGCOM in the garage, and I intended to come back, forgot, and the interior light, and parking lights (which were on) drained the battery.
After two hours on a charger, it started right up. I hope it's just that for you. Batteries can only be expected to last about 5 years. Sometimes less in severe duty (really cold or really hot) conditions. I am going to replace mine next Fall before the winter strikes. Fortunately for me, I have a 100 amp load tester, and a hydrometer, and optical refractometer. This is because I own a travel trailer and have to care for two 6 volt golf cart batteries. My charged Touareg battery measured fine after a full charge (later that weekend I put it on the charger overnight to make sure it was 100%).