You didn't mention make and model, but first guess to me is the timing belt (if equipped) has slippel or broken. There could be some engine damage if the engine is an interference type. At minimum there could be bent valves. That would explain why you couldn't crank it by hand.
Ok, my son was driving his car home and it started to sputter and he said it backfired through the exhaust and had no power. He stated he was going up a hill and had the gas pedal all the way down and the fastest he could get it to go was 25 MPH. He pulled to the side of the road and turned the car off. Upon trying to start it, it only made a loud clunking noise and the engine would not turn over.
I checked the starter and battery both are fine, when you try to start the car the start makes a loud clunk as if it is trying to turn the motor but can't. i tried turning the motor over by hand and it will only turn about 1/2 to 3/4 the way around and stops, you cannot turn it any further. When you try to start it, the engine turns over about that same distance back to where I started to turn the engine by hand from. Its as if the engine is catching on something. Does this mean its seized or something broke internally??

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