You need to pull the fault code(s) with a VAG-COM or try a local parts store that will read OBC-2 fault codes for you for free. (most will do it free here in the USA) The OBC-2 readers may or may not be able to tell you the problem..but it is worth a try.
Your problem could be many things, a ignition switch failing..a plugged up fuel filter..a dying fuel pump..a bad electrical connector..the list goes on. Having a VCDS VAG-COM fault code helps.
If I were to guess...the ignition switch would be first..easy enough to change and get an OEM switch from the dealer...the camshaft sensor can cause this problem..the plug connector can break and hang loose and the sensor itself can become faulty..the same deal with the crankshaft sensor. quattroworld.com Forums: C4 no start
If you suspect the cam sensor (a likely suspect) you can get a new one from Hans for $16..I'm running one in my A6 now without issues.
078905161C $16 VW Audi Camshaft Position Sensor 80/90 100 A4 A6/S6 Cabriolet V6