First things first: open the machine and pull out its guts.
Next, the arm. If it's come unscrewed, screw it back in; if it broke or fell off, refit it.
One thing is critical: the machine must be in "OFF" mode so the servo motor is in the correct position — the arm's axle must sit perpendicular to the top cover.
The only slightly tricky part is fitting the circuit board back into the slots (as you can see).
Even if the arm is properly positioned, it's worth recalibrating so the software can fine-tune the motor position (not strictly required, though). For this you'll need Chrome or Edge (no other browser will work) and a cable to connect the machine to your computer. Then follow the steps in the video — once the test has run, the machine is calibrated.