Here's what I mean, for the ring to spin the bolts must've become loose enough for the teeth to disengage, make sure they're in line for re-tightening and make sure they're well tightened or use super-glue on the bolts if you want to be sure of it not happening again.

There will be a more thorough picture series for the gearbox on the new website, currently waiting for some decent sunlight to take the next round of pictures (much cheaper than photographic lighting!).