firstly were the gears ok before the clutch change ?
if yes it could be the clutch dragging slightly / faulty clutch plate / incorrect adjustment
if the fault was before the clutch change -
& If you can't select forward gears when the engine is off ...
it is possible that the reverse gear bush has moved & is holding the car in reverse slightly - not engaging gear , but enough to stop other gears selecting
I had it on a mini a few years ago , gearbox needed a strip down sadly
I would go thru all other possible problems before checking for this
check things like the roll pin coming loose , as above - the gear selector shaft boot fouling the selector shaft joiner tube etc
also check the gear stick pivot box for wear / something fouling the shaft , even the reverse lift gate on the plate on the bottom of the pivot box (spot welds break)
best of luck
Edited by sledgehammer, 18 August 2018 - 11:18 PM.