Hi,
I'd guess its something on the lines of your gear lever isn't pulling the box out of reverse. So box stays in reverse, you select another gear (which I don't think should be possible, cos detent balls are there to stop that ...... but I've had it happen a couple of times). If gearbox is in TWO gears at same time NOTHING will turn.
My first move would be to try and waggle the thingy that the bottom of the gear lever pokes into. I think you can just see it without removing any bodywork. If it waggles, you may be lucky, and its only a couple of loose nuts, or bolts, or both. Next wish is that its only a stripped thead in the clutch housing. Worst is a worn selector, or a selector retaining screw come loose ...... inside the box. Had that on a Mini, sod of a job.
602