Hi
It is not normal for your Apple Watch to turn off unexpectedly.
Whilst I can't say why it is doing so, the following steps may help to prevent the issue from recurring:
Restart both your iPhone and your Apple Watch, turning both off together, then restarting your iPhone first:
- Restart your iPhone, iPad, or iPod touch - Apple Support
- Restart your Apple Watch - Apple Support
- If the issue persists, next close all open apps on your iPhone and then restart both devices again.
Test by removing apps:
- Remove third-party apps from the Dock and/or from your Apple Watch and then monitor performance after adding them back, one at a time:
- Remove watch face complications and monitor performance as you add them back:
Unpair and re-pair your Apple Watch:
- The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app.
- Initially choose to Restore from Backup during set up (most data and settings will be restored, with some exceptions, such as Apple Pay cards, passcode).
- If the issue persists, consider instead unpairing and setting it up as a new watch.
- Unpair your Apple Watch and iPhone - Apple Support
- Set up your Apple Watch - Apple Support
If the issue then still persists, I suggest that you contact Apple Support (mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider with a view to having it checked and, if recommended, serviced:
Apple Watch Service Answer Center - Apple Support
Be sure to keep your Apple Watch within the normal operating and storage temperature ranges and also keep in mind that it is normal for Apple Watch, the charging cable and the USB power adapter to become warm when charging your Apple Watch:
Keep Apple Watch within acceptable operating temperatures - Apple Support