can you move the gear stick from side to side, and then move it up and down,?
did you ever remove the linkage top cover it has 4 bolts on the top of the gearbox?
This happened to me once and another person on here last week.
When you unbolt the front selector cable and lever instead of disconnecting the ball joint. The selector fork drops down and can move out of assignment.
As mentioned above. Remove the top cover from the gearbox and centre the selector. Seal the cover with Blue Hylomar sealant (very useful for cam cover etc too) and be careful to keep the shaft from dropping and moving when reconnecting the lever.