How can I fix my chain saw?

I have an Oregon electric chain saw 1800 watt. PWR1800CSD. Serial No. 04894.

The motor is running OK but it is not turning the chain.

With the chain removed the motor is turning the drive sprocket, but when the chain is replaced the motor runs but the chain does not move.

With the chain removed and the power switched off there is about one turn of free play on the drive sprocket which I can turn with my fingers.

The failure occurred suddenly, just after I had cut down a sapling about 3" in diameter. I noticed it when I tried to cut another branch.

With the chain in place and the motor off I can turn the chain and motor by pushing the chain along the surface of a log.

Moving the kick-back safety lever back and forward does not solve the problem.

I suspect that some safety device has operated to lock the chain drive but has not returned to the unlock position.

Please advise what the cause might be and how I can fix it.

Thanks,

Dennis