The model/year will certainly help diagnose the problem.
But from what you describe it sounds like you allowed the shift shaft to come too far out of the case which allowed the spring loaded cam follower to pop out of the end of the propeller shaft. When that happens you can not get the cam shifter, which is attached to the bottom of the shift shaft back in it's proper location which allows you to shift between forward, neutral and reverse. (the cam follower is like a large (pin) and when it pops out it blocks the front of the gearcase where the end of the shift shaft "should be" - you can't move it backwards, back where it belongs when this happens without disassembly).
The "natural" position is reverse and if you are stuck in it, the above is almost always the case.
If that is what happened then you have to pull the prop shaft out (so tear down the gearcase) to get it set up again. I have been there myself on more occasions than I can really remember. If you can get the bearing carrier (on the propshaft) out then it's not a terrible job. But on older motors that is usually the "booger" that turns the job into a nightmare...