Without a further explanation of what the OP means by “the engine is free spinning”, it’s sort of hard to say what the problem is.
If by that the OP means they hear a buzzing or multiple-clicking noise when they turn the key to “start” but the starter motor fails to crank the engine, that’s often caused by an electrical system problem of one kind or another. Bad battery or bad connections or failing starter motor. What happens is power is applied from the battery to the starter motor, which causes the starter relay/solenoid to engage, current starts to flow to the starter motor, then the voltage immediately drops due to an electrical problem causing a high resistance, which disengages the relay/solenoid. That process repeating itself causes a buzzing or clicking noise.
If that’s what’s happening, the first thing I’d do is clean and re-tighten the connections at the battery. Next would be a battery load test and a charging system test. And check for a failing ignition switch.