why has my fox rear shocks adjusting lever popped out?
I'll assume that your adjustor knob looks like this: 


If you look closelly to that image, over the "2" you'll see a triangle. Over the triangle you'll see a tiny hole. In that hole there is an alen bolt, (maybe 1mm or 1.5mm) which retains the knob in its place. That bolt must have been lose so the knob fell. The metal ball is just there to provide the click sound and feel for the compression or rebound adjustor.

So, fully undo the bolt, place the knob in it's place, tighten the bolt (not very much), and you are good to go. Test by rotating both directions and expect to hear and feel the clicks. Make sure you return the settings to their original position. If unsure, consult with someone who knows, your LBS, or ask here.

This exact mechanism exists in almost all compression and rebound knobs in common suspensions. e.g  and