You need to check you're shifter shaft out; pull off the right crankcase/clutch cover and look behind and above the clutch assembly at about the 11 O'clock position, you'll see the end of the shift drum and the actuating arm w/ spring of the shifter shaft assembly.You'll be able to check it by moving the shifter and seeing what takes place at that 11 AM position with the shift drum.The shifter shaft is either snapped at some place where it goes through the engine cases...or the actuating arms and spring are somehow fouled up;I'd vote for the latter. You'll see it, and then the clutch will have to come out to repair or replace those parts;one of the little small dowel pins may have fallen out of the end of the shift drum after the Large phillips screw holding that "star"looking piece on the end of the shift drum has loosened up...it could be any one of those things.
It's Not a complicated job....Enjoy