Apple Watch screen fell off

Your picture suggests the screen has debonded without breaking the glass. DIY repair is not really an option, and it is possible the battery has swollen to force the electronics module to pop off. Treat it gently, don’t meddle just Contact Apple Support for advice and repair options.

If you have a first generation watch see:

https://9to5mac.com/2017/04/28/apple-watch-expanded-swollen-battery-repair-coverage-warranty/