Engine cranks but won't start??
An engine needs gas, spark and compression to start up.
We will assume timing chain didn't break, because you said engine cranks, it wouldn't if a piston hit an open valve, so we will assume 150psi of compression.
Fuel and spark, well I guess the easiest thing to do would be to get some Starting Fluid(ether in a spray can) at the auto parts store, and spray some into the engine and see if it fires up.
You can remove a larger vacuum line like the PCV hose or Power brake hose, spray fluid into intake then replace hose.
If it fires up then dies, you have no gas getting to the engine.
If it doesn't fire up then you have no spark.