gearshift stuck in 4th gear

My '90 626GT has a stuck gearshift. Stays in fourth and won't go into other gears, but clutch seems to work OK because when I depress it disengages 4th gear, but gearshift still frozen in place. Ideas?

check the bushes under the gear lever for excess play...otherwise your 'box might be no good...they dont last more than about 200,000kms...but before you change it out, disconnect the linkage at the box and move it into other gears by hand.