I presume you mean the rotor and if so its secured to the autowind bridge by a single screw. If that didn't fall out the back when you opened the watch its still in the movement somewhere, look very carefully and if you spot it remove it even more carefully, there's a lot in there that is easily damaged. The fact that the rotor has come off doesn't unfortunately address the issue of the chrono hand shifting on its post, the fix for that is to remove the hand and re-set it.