Won't crank?

When you turn the engine over by hand, are going in the normal direction or are you turning the engine backwards from its normal rotation? If you have a hydrolock (coolant on top of piston which will not compress at TDC compression stroke -- thus the engine locks down) and you turn it backwards, you may being pushing the water out of the cylinder on the exhaust stroke and when you try to crank it again all is well.

Is it low on coolant?