Chosen Solution

The back of my Apple Watch fell off. It still functions. The heart rate monitor needs to be plugged back in and there needs to be some kind of adhesive or seal to seal down the rear round ceramic piece back onto the watch. Are there any parts available for this? thanks.

Surprisingly Apple has an extended warranty program for this problem! Bring your watch in to an Apple Store for the free repair. For reference: Original Apple Watch gets extended service coverage

Brought mine to the Apple Store. They are no longer repairing this issue under warranty. I missed the extended warranty repair window by 6 months.