Stolen Apple watch
My Apple Watch has been stolen and I can’t find it on find my Apple Watch because it has been turned offline, what shall I do?
Apple Watch
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The only way it could have "been turned offline" is if the thief knew your Apple ID & password. If you suspect he did, then you should change your Apple D password immediately.
Change your Apple ID password - Apple Support'
BTW, it may simply be powered off. See the "If you don't see your Apple Watch on the map" section in:
If your Apple Watch is lost or stolen - Apple Support