Starter motor spinning but not turning over engine

It sounds as if the starter is turning but not engaging so I'd suspect the clutch or the gear is loose or without teeth, but even without teeth you'd likely hear unpleasant grinding noises. I'd expect the clutch and starter are all one unit. (They are on a car where space is less critical than a bike.) Try giving the starter a bang with the handle of a screwdriver or hammer to see if it frees the gears to engage while you're trying to start it. If it starts, replace the starter. You should get a re-core charge if you don't bring in the old starter. (You do with cars.)

The one way clutch inside the motor is slipping. On the back side flywheel is a one way clutch that grabs the starter gear when the starter motor spins. It is designed to let go of the gear once the motor fires up. If the mating surface of the one way clutch and the starter gear becomes rough or chattered, it will no longer grab and turn the motor over. To repair this you will have to take off the crankcase cover and pull off the fly wheel to replace the one way clutch and the starter gear.

The starter plugs into a small gear that spins the starter gear using a thin chain. On the side of the starter gear is a smooth surface. If the smooth surface becomes rough or chattered the one -way clutch will not grab hold of it and spin the motor. You will need to open the motor and replace the starter gear and possibly the one-way clutch. Then your bike will turn over again.