Check your shift cable
Check the cable that runs from your cockpit shifter to the shifter on the transmission behind your engine. The cable may be broken, or the link from the cable to the lever, or the connections from cable to linkage to lever.
Same thing just happened to me. The linkage corroded where it connects to the cable, and the screw rod on the end of the cable pulled out of the linkage. A simple fix - a new linkage is on order from Max Boring.
Hope this helps.