97 sunfire shift problem

Hi, I was hoping someone could help me. My sons' 97 sunfire is stuck in 3rd gear. He was shifting from 3rd to 4th gear and it stayed in 3rd. The shifter moves freely up and down only, you can feel that it is not shifting anything, but doesnt move side to side. I suspect something has let go iwith the shifter linkage but I thought I'd ask before I tear into it. Any help is appreciated.
Thanks Jim

Most 97 Sunfires have a hydraulic clutch sytem. When the hydraulic oil(brake fluid) runs low the pressure in the clutch and the shifting of gears is affected. Simply fill the resevoir by the mastercylinder and you should be shifting like normal in no time. Be aware that you will not need too much, as the resevoir is very small and the cap sinks into the resevoir.