My dad's Yamaha 200 did the same thing a few years ago. We were riding and he downshifted into 1st and then it wouldn't shift, up or down. It turned out to be his shift fork shaft that is in the trany. The shaft bent just enough to keep the forks from sliding up and down it. The shaft cost $10 from Yamaha (plus gasket kit) but that meant that we had to take apart the whole engine.
-hope this helps, Andrew
1984 Yamaha YT 60 Tri Zinger(current project)
1983 Yamaha YTM 200 (rear suspension kit)..... long term project
2004 BMW R1150R
1972 Honda CT 70
2007 Lifan 110