well u should check your fuses before you pay any money for an audi tech to check it out, other then that it can be a special code that u may need, every audi has it for secuirty reasons, you can only get that code at the dealership, and they have to take out the stereo,and plug it to a computer and get the code, that cost about $150.
and dont try to take the radio out yourself, you need a specail key inserted to a hidden location on the stereo.
, but yea check your fused first then resort to and audi tech