You will need to take things apart, the lower part on the range lever which engages the shift fork may be broken off or just badly worn. It will probably involve removing the top cover. I had a little trouble with the gear selector jumping out of the shifter fork, and was able to put it back into place by pulling up the boot, driving out the pivot pin, raising it up against spring pressure and reinstalling. It hasn't given any more trouble.
Maybe yours will be an easy fix, try the easy way first. Good luck.