A3 not starting
 

Hiya my A3 TDI sport is on a 2000 plate. I don't know if anyone has come across this problem before. When i try to start it its as if only the top half of the engine is turning. I have checked the timing belt. Its ok. Someone said that the valves are sticking and this is why there is no compression in the engine. Anyone any thoughts?

Thanks in advance