I've had the same problem. It was the starter on mine. You should carry a hammer just in case that happens again; you would just hit the starter and the starter solenoid if it does not want to start. If you do find that it is the starter then buy yourself the solelnoid only. I did that on mine and it only cost me 23 bucks. Search Auto Barn I believe and they have the rebuild kit.

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Audi TT