Agree with the above. Look under the coolant header tank and try and see the ends of the gearshift cables. Get you assistant to wiggle the stick and see if anything moves down there. If the cables are still firmly attached and the stick won't move then you have an internal fault. Luckily, this will almost certainly not be right inside the gearbox, it will be in the selector housing.
If you need to move the car and it is actually stuck in gear then unbolt the selector housing (no oil is lost) and you'll see this:
You need to line up all the selector sliders (use a large, clean screwdriver) and that is neutral. Have a quick test with an assistant having loosely reassembled and check you can get 1-4 + reverse without sticking then tighten the bolts and take the car to where it is needed.
The fix will be one of two things:
1) 5th has a little cam which stops you going from 5th straight to reverse by accident. After I DIY assembled mine once, this was sometimes popping in and sticking it in 5th. I think cable adjustment can affect this.
2) There is a fault with 5th itself inside the workings. This is obviously worse, but 5th is under a cover at the end of the g'box and doesn't necessitate gearbox removal:
Get your local gearbox specialist in there and let them inspect the synchros etc to see where the fault lies.
So in summary, probably and hopefully a cable end snapped. If not, probably a minor issue in the selctor housing. Failing that, you need a gearbox specialist but it isn't the end of the world.
Candles.