Gearbox problems

Hi all,
Just lost all gears apart from 4th, gear lever all wobbly. Took the top off the box and the ball and plastic cage off the end of the gear lever were nowhere to be seen. Happened to me about 6 months ago but then the ball was still in the selector, just the plastic cage and metal clip had vanished. Anyone know what the reason is for this to happen again.? Just hope I can recover the ball from the inspection cover?
Paul

Duh, no inspection plate on the main box. Drained the oil, no bits came out. Had a fish around from the top ,found the ball, cage and clip! Must remember to put a lottery ticket on! Part of the cage was missing and the rest of the cage chewed up. I fitted the slick shift mod when I changed it last time the cage disappeared, wonder if that's why the ball drops out now if the cage is damaged as I think the slick shift raises the selector shaft to increase the throw slightly?
Paul