How do I turn Apple Watch on? After charging
How do I turn on my apple SE watch
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Press the power button on the side of the Watch. It is the rectangular button below the Digital Crown. Although, if you were charging the Watch, it should have been on. If it had gone dead and you charged it, then it should have turned on when it had enough charge to start.
See if this helps, If your Apple Watch won't charge or it won't turn on - Apple Support