Possible that the starter pinion gear has come off or the pinion itself is not throwing out to engage on the ring gear. You may have to take the starter out to check and confirm. The solenoid which sits above the starter motor operates the pinion. It throws out the pinion shaft to engage the gear and the starter motor itself, spins it. Once the engine has started, the solenoid retracts the shaft and gear back and away from the ring gear.
