Hi All: I have a Merc classic 50 that seems to be stuck in forward. I searched the forums but either didn't find what I needed or didn't make sense of it (sorry).
Perhaps someone has removed the lower unit, like to do a water pump impeller service, and not aligned the shifting splines correctly when they put it back together.
Beyond that there are a number of other possible issues that could cause loss of shift control.
Such as, but not limited to,
Cables, cable connections and supports, controller, broken or rusted away components, human error from previous repairs, etc.
I suggest, if you are unable to look it over and see if all the mechanisms are working, you have it looked at professionally.
Some shops might give you a free estimate, otherwise expect to pay $80-$100+ an hour for services, plus parts.
Me, I'd start by looking to see that the appropriate cable is actuating the mechanism under the hood. And if that looks good, I'd be dropping the lower unit and seeing if it were functional.
It is possible that something broke or rusted away between the two areas, but you, or a mechanic would have to delve in to see.