James, if the OP has all other gears, if it drives normally in other gears ,most likely a lost synchro spring which holds the 3 slipper pads in place, these rock like seesaw depending on 3rd or 4th selected and hold the synchro in gear...
If spring comes out slipper pads can come out of line jamming that gear only way to fix is to strip box,the only other likely thing is very common where brgs wear out letting input shaft drop putting it out of alignment with mainshaft but would be noisy in all gears and will quickly loose all drive ,if you drop oil you will probably find parts of 10 cm c shaped wire spring ...? idea !