If you look down at the trans where the cables attach to the shift tower, the cable sheaths also attach to the transmission with black metal sheet metal clips. Sometimes the clips come off and the cables are loose and it is very hard to get into and out of gear. If that is the case then you need to get new clips. If that is not the case then more than likely, you need to replace the shift cables.
Remove the clips that attach the cables to the shift tower and you should be able to run the transmiaaion through the 5 forward gears and reverse...if that is OK the it's the cables...if not then it is internal to the trans.