Audi a6

Put your foot on car gas pedal while turning the key hold foot there until it starts a mechanic showed me that it's not getting enough gas. It's either your fuel filter or your pump. Especially if it is fuel injected. If not go down the line with all sensor that has flows through.

If your battery is weak have it load tested, replace it if its weak. Change your Battery terminal & terminal wires, tighten loose connection (including starter & loose grounds to frame from engine & battery). Terminal wires can be corroded under the insulation unseen. Grounds to frame can become rusty at the frame side and be little effective at grounding. If its more mechanical engine problems confirm if fuel or spark is present in-order to know which route to take.

Check your fuses & relays, fuses are under dash & hood may be some in rear cargo area behind/under trim. Relays: some are under hood, a few under dash and some may be near the part it functions with. If your check engine light is on have the computer scanned for codes.