Changed my rear brakes on my 1998 Audi quattro A6 and now it won't start, how do I fix this?

do you hear a clicking noise when you turn the key? if not check your starter. Putting the car down could a just jiggled a bad wire loose. If you have the original starter in that car it more then likely had the wire between the points and solenoids rot off its very common with audi's.

Those two issues aren't related to each other. Is your audi an automatic or stick? If it's a automatic it might be the neutral safety switch is faulty. Try starting it in neutral. Is it turning over but not starting? Is it getting spark? Please try to give us as much information as you can, thanks. Check all your connections from battery to starter. When you try to crank it do you see sparks smoke from battery terminals?