The following comments are made assuming the same design as on the Axis 24.2.
On my Axis, the little wheel at the end of the control arm slides in at the end of the track. I don't know of a way to get it there without removing the crank mechanism from the frame which, on my rig involves drilling out two pop rivets and replacing them when it is reassembled.
I can't see how it would have come out of the track unless it broke from the little wheel. If the arm came out somewhere along the track, you may have a bigger problem.
Perhaps more details or pictures would help. Others may have more experience or ideas.