gear box

hi, can anyone help? I have a disco 1 1992 200, I was going up hill in third gear, I went to put it in forth but it would not go in, so I put it fifth but that was to much for it, when I tried to come out of fifth it was stuck and still is the thing is the gear stick moves as though it in neutral. please help.
thanks Richard.

check grub screw on the selector shaft yoke under gear stick turret

just want to say thanks to jamesmartin , you were right mate it was the grub screw.
thanks again mate. and good look with everything.:high5: