G'day Let's start with the simple things first.

Can you turn the engine over by hand. Does it feel smooth?

If it does not, pull the propellor off and see if the prop driver (the metal bit behind the propellor) is cracked. This is what XJET is talking about. These things sit on a tapered collet and sometimes they split and then when you tighten the prop, it forces the damaged prop driver back against the crank case and the engine won't turn over.