Usually one of 3 things. Spun engine coupler (engine out to fix), spun propeller hub (replace/repair propeller) or a broken drive shaft or stripped gear (replace/repair as required)...
To check the prop hub, mark the propeller outer and inner hubs. Then take the boat back to the water and try it in gear. If the marks have moved, replace/repair the propeller...
To check the engine coupler, remove the drive and have a look at the yoke splines and the coupler splines (coupler splines can be seen in through the gimbal bearing).
to check gears and shafts, with the drive off, put it in forward gear, hold the prop and try to turn the input yoke. If the yoke turns without the prop moving, split the drive. Check the drive shaft and if that's ok, take the top off the drive and have a look at the top gear set.....
Chris......