I have a gearbox with exactly the same symptoms, it is a super-long 4th gear, six rib, cog box special, it would be a mega result if this is the issue. Must get to the garage once I'm off dog watch.
If nothing was broken, but it had just locked it self out, then looks like it can be unlocked through the filling spout without removing the gearbox (stubby screw driver). Worth knowing and good trick if it worked - wish I'd come across this 2 gearboxes ago.
