Same happened to me. I knew it was just out of warranty and knew I'd have to pay for repair , at probably the original cost of the watch. After googling same problem on this and other watches, general consensus was to carefully glue back with E6000 glue. I meticulously cleaned the insides of dust and muck and carefully applied the glue - not an easy process. It has held up well and my Active 2 is functioning the same as before the back fell off. However the ECG does not work (did for a few weeks, after the Sept. 2020 update), but not anymore, since Nov. 2020 ... but that's another samsung watch active2 thread to follow. Turns out after giving all my Galaxy Watch Active 2 and Galaxy Note 5 info, i.e.,model and serial numbers, system versions, etc. to the "ECG Team", their diagnosis was that my Active2 device was " a non-U.S. model or a model that is not manufactured for use in the U.S.
We are sorry to inform you that the ECG feature will not work in your device. " Ridiculous - I purchased at Costco, NJ, USA Nov. 2019 - So non-U.S. Samsung model devices that are Not Manufactured for Use in the U.S., are being sold all over the U.S.??!! I can live without the ECG, but it's all sooo annoying. Does it bother anyone that a sport activity watch that monitors swimming IS NOT WARRANTIED TO BE WATERPROOF OR EVEN SWEATPROOF? THE SENSOR COVER SHOULD NOT BE FALLING OFF AND OWNERS SHOULD NOT BE PENALIZED FOR WATER DAMAGE ON A SUPPOSEDLY WATERPRROF DEVICE.