does the shift lever move? If so, the bottom may have snapped off.....Take out the panel in the floor, remove the rubber boot, press and twist the retaining plate, and pull the bits out.
If the lever is locked, it may be the shifter sheet that has worn.
Same procedure, but under the shift lever ball, you will find a plastic bushing that the shifter sits in. These wear and can lock the box.
If its non of the above, then its shagged and needs to come apart!
:D
