Hi
If you used a third-party app to track your sleep, then it is possible that the app's operation involved sufficient battery drain to fully discharge your Apple Watch's battery.
I suggest that you check the developer's support resources to confirm that you are using their app as they advise and also to check for any known issues, if necessary including by contacting the app developer.
If you have not yet done so, then it may also help to update your iPhone, Apple Watch and all third-party apps to the latest software versions.
In respect of your Apple Watch now being dead, I suggest that you follow the instructions here, including contacting Apple Support (mail-in service may be available, if required), making a Genius Bar reservation or visiting an Apple Authorised Service Provider if necessary:
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Why did I wake up to a dead Apple Watch?