@Ravan_Caw: I'm really sorry to hear that you have encountered this issue with your Galaxy watch Active. If the sensor on the back has come away, then this will need to be repaired by a Samsung engineer at one of our authorised Service Centres. With this in mind, please reach out to the relevant support team in your region by heading to samsung.com > Scroll to the bottom of the page > Contact us, and an advisor will be able to make the necessary arrangements for you.
Please be aware that if the device is over 24 months old, or the issue is deemed to be the result of physical damage, then a charge will be applicable to cover the cost of the service.
You can buy a new sensor from Amazon and install it yourself with YouTube instructions. However, there is a new Samsung problem with Samsung Pay app constantly loading which is also killing the battery life, AND SAMSUNG KNOWS it. WHERE is the software fix? If they don't provide a software fix for this, I won't buy another Samsung product either.