Hello, welcome to JustAnswer.
Chances are pretty good that you have either a spun (or loose) prop or your engine coupler is bad.
To figure out which, put the drive in forward gear and PULL THE SAFETY LANYARD.
then go back to the prop. (is it loose?) If it is not, try to turn the propeller backwards (opposite the direction it would turn if it was pushing the boat forward) try very hard. If you can, is the nut on the propshaft turning?
If the nut is turning with the propeller, have a helper look down behind the motor at the outdrive's input shaft that comes out of the engine coupler and goes thru the transom assembly. Is it turning?
If the propeller turns, but the propshaft does not, the propeller hub is bad.
If the propshaft turns with the propeller and your helper can see or feel the input shaft turning the engine coupler is bad.
If the input shaft does not turn and the propeller shaft spins with the propeller the problem is inside the out drive. (or the drive is in neutral, put it in gear and try again)