Originally Posted by Sprode
I looked at the gears, and they are all spinning except for the bottom bull gear in the center. It looks like it may have been supposed to be contacting with the spur gear in the rear gear-stack. Could those two have come out of alignment somehow?
I see in your pic of the blue S107, I assume that's yours, that the center shaft and main gear have sliiped below the sprocket on the middle gear. You should be able to press it back up. Press down on the top of the main head and try to pry up with perhaps a micro screwdriver under that main gear. You could still have a broken main shaft as well, but I would try that first.
If you have a broken inner shaft though I think the head would just come off if you took the flybar links off.
I would order a full shaft and gear set with your next parts order, just in case.