When you say it tries to start but won't do you mean the engine is turning and won't start or do you mean the engine is trying to turn but won't.

There is a big difference between the two:

1) Engine is turning but won't start - ignition, fuel or faulty sensor etc.
2) Engine is trying to turn - Coolant may be in the cylinders AKA hydrolocked.

#2 is worst case scenario. A hydrolocked condition requires an experienced mechanic to dissasemble the top of the engine and replace faulty seals.