It seems what the adhesive was broken somehow, buy some B-7000 [link|https://www.amazon.com/B7000-110ML-Adhesive-Multi-Function-Transperant/dp/B09DL5P2M6|here (amazon)|new_window=true] or at your local walmart
Add some of the glue around the edges of the watch where it fell off, put the back on and let sit with a clamp (or any other way to keep it secure in place) for ~24 hours.
But first check that nothing is broken inside and the charger and charging wires aren't damaged or will be damaged by the new glue.