disco 1 200tdi gearbox probs

Evening gents my 200tdi has problem .it's lost low ratio diff lock high is there .just can't fathom it used it one day went to use it the next light comes on on the dash but no drive at all move the diff lock lever to high and we have drive take diff locks out drives fine .no funny noises .any ideas ???? What am I missing here ...anyone ??

check the linkage and grub screw in hi/lo housing on t/box

Where abouts is it located on the side or top regards tony

linkage is on the side, grub screw is under top housing 6 x bolts to remove it you should be able to feel if housing shafts got excess play without removing it as you can with top lever housing