Lanski,

The simplest thing wrong could simply be that the bolts, etc, fastening the
selector linkage together, under the car have come loose, look underneath
while an assistant is moving the gear stick back and forth, and observe
whether the motion is being correctly translated to the gearbox, (check the
the small input shaft on the gearbox actually moves, or not) if this is the
problem, then it's very simple to fix, you could even do it yourself in
about 10 minutes, post back what is loose or not moving, but don't just
tighten stuff up or the gear you pick with the stick, won't be the gear you
get. :-)
Alternatively and I can't imagine this being the problem, the valves and the
detents that the selector shaft on the gearbox operates have failed
somehow, or the selector input shaft has sheared, both of which would be
very costly repairs beyond the average diy'er.

TTFN

Ken Phillips