Have someone shift with the car off while you look under the hood. On top of the transmission is a plastic shifter bracket, this can break. If it breaks you can't shift.
It might just be a a shitfter linkage alignment problem so try that as well.
Or a bubble in the clutch air line. Check for low clutch/brake fluid. Or try double pumping the clutch before trying to shift. It's probably one of these 3 solutions.
Any updates? I believe it's a 13 or 10 mm bolt that holds it on. Just because of age it deteriorates and goes out of alignment. Make sure the clutch fluid is changed every 2 years too since it absorbs water.
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