It is possible that you have something jammed in the linkage as Linc mentioned, It is also possible that the shifter has gotten the external shifting forks mis-aligned. I did that once on a Century 2535 (Branson 3510),not sure how, but the shifter would not move. It freed up while I was fooling with the lever and every thing went back to working. Take a look at the mechanism at the base of the shifting lever. It probably is something simple. Good Luck. W. Jones