Motorcycle Repair: stuck shifter, yamaha 250, michael hull

Question
Thank you for reading my question. I have a 1975 Yamaha 250 Enduro, It seems to have stuck in 2nd gear, I was just riding along and started to down shift and went from 3rd down to 2nd and that was it.  It will run and drive but you have to take off in 2nd gear, it makes no unusual noises and the shifter and shaft moves freely, it just doesnt click up or down.  I would appreciate some advice.      
thank you
michael hull

Answer
Michael,
  At this point I would just check three things before resorting to splitting the case.  1) the shifter is slipping slightly on it's shaft due to worn splines.  2) you are very low or drained of gear box oil.  or 3) you have a dragging clutch and it needs to be adjusted to the proper amount of free play.
 Other than this you are dealing w/ an internal gearbox ailment, most likely a worn or broken shifting fork.

Mike