1976 240D Manual Trans Stuck in Gear ??

Out of the blue, I am driving home and the gearshifter will not allow me to change gears, more like to get out of what I believe is either 2nd or 3rd gear. Anyone have an answer on what it could be? I don't want to have to get it towed and fork over $$$ to a mechanic, but if this isn't something I can fix myself, I guess that I will have to.

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