Keep the gear lever, strip the rest down :)
I would check the clutch operation first, It sounds like the clutch is not disengaging properly and not allowing you to change gear.
After that, Check your gearbox oil too... May be worth draining it out and looking for bits of gear, but I think it's more likely to be a clutch issue.
Is it a Hydraulic clutch or mechanical on a series?? (No joke, I've not looked at the mechanics of a Landy 'cept the blasted handbrake)