There are plenty of reasons that a gearbox won't go into the selected gear from a broken selector cable or disconnected linkage to a problem with the gearbox itself. The video below talks about this situation with instructions for various repairs below that.

How to replace a control cable with a new one.

This topic coverse removing an old control cable and installing a new one.

Try to repair your steering cable

It is sometimes possible to free up a siezed steering cable by using compressed air to force oil down inside the casing. To do this you will require the use of an air compressor, although it can be quite small such as one for inflating car tyres.