Spinning the prop is the first thing to do - so you have checked that the gears are free to engage.
If you haven't changed your impeller recently there may be a problem in the
gearbox, then you probably should undo the gearshift linkage and see if that frees it up. The linkage is held together with a long nut. You have to undo about 4 bolts too - there may be a bolt hidden (I can't remember - the 25hp has one under a rubber grommet).
You should remove the leg every two years at a minimum - having a st uck leg ruins an engine as you are one impeller from
cooking it so take this opportunity to check the impeller anyway.
If you removed the leg (and the
gearbox linkage works fine) then the problem is probably in the linkage up top. It could be that
corrosion has done its thing there. Be careful as many of these
parts are cast alloy and rather brittle -
work gently rather than roughly.
On some motors there are little plastic links that can fall off the gearshift, check that they are not fouling the flywheel or insides somehow.
Be careful and gentle. If it is siezed in the gearbox that is bad but you can get another gearbox. Get the leg off and give the shaft a nice lick of grease - it sounds as though the
motor needs it.