Collect all the bits for the gear cluster and... you do remember what order they go in right?
Push the potentiometer back into place carefully. Don't try to rotate the gears at this point, it's highly likely that you will damage them if you do.
Replace the PCB as you found it.
Make sure that the servo wires aren't pinched... and then re-insert the screws.
Now... all you need is a project to put them in!