Start with taking the left side cover off..there is a small pin that provides the shifter return spring an anchor point.
If it is sticking down the pin is likely broken off (common issue)..though it seems like you went to neutral you really didn't. It's likely in the gear you just downshifted into, but the shifter being stuck in this position is keeping your clutch disengaged making you think you're in N!
The pin is about $12 and is still dealer available..then replace the side cover gasket and your oil of course since you have to drain it to get the cover off. Try to find the piece of pin while you are in there..no point leaving it in the bottom to cause more damage later..
My money is on this being your problem..fairly simple and cheap.
Good luck..
Current toys..
1986 Honda 350X..trail bomb!
1985 Honda 250SX..my main mudder
1985 Honda 250ES..Back in Black Trike
Current non-trike toys:
1990 Honda TRX300FW
1995 Seadoo GTX
1998 Polaris Indy Lite 340(Nearly new looking)
1998 Polaris Touring 500
1998 Club Car (electric)