Is the gear selector free? Like stirring a stew pot but nothing is engaging? If the selector bush has worn out or straight up disintegrated or the selector lever housing isn't screwed down, the selector lever can pop out of the linkage and drop the selector fork out of it's cluster. If that is the case you won't need to pull the gearbox apart to fix it. You should be able to manipulate it back onto the fork and re-engage it. You'll feel it, just takes patience to figure out where the selector rods are going. You will need to do this with the gear lever off the gearbox so you can get to the selector linkage and jiggle it around.