Your asking a very general question my friend........
Tough to answer with so little info.
Get a service manual or download one. Take the cover off (probably the right cover) that shows you the EXTERNAL shifting goodies and lots of times you get lucky and find a broken return spring or spring post etc etc etc.
Unless you beat the chit out of it (only you know) it's a Honda and very rarely is it an internal problem (meaning inside the center cases that NO ONE wants to touch)
I've had many Honda's, beat them to death (but I beat my machines with a brain - if anyone can understand that!!) I have never lost a tranny gear and or fork or shift drum etc etc which is good news for you!
Ask MonroeMike for a manual, he's the best
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