You need to check for fault codes in the first instance, either vag-com or I bought a VW Code Reader off ebay which gives you the basic codes.
Its difficult to give an exact answer without the code, however it could be the G28 Speed Sensor, someone had same
problem on here. Its a magnet type and when magnets get hot they can lose some of their magnetism and affect the sensors reliability hence stopping the engine starting until its cooled down.
It could also be the temperature sensor. But as I said best to get the codes read in the first instance.