Apple Watch Stuck on pairing screen/won’t turn on

Does your iPhone show the message, that the watch as to be restored?

If your Apple Watch shows an iPhone and a watch or a red “!” - Apple Support

Also have a look at the troubleshooting mentioned in these articles:

If your Apple Watch won't charge or it won't turn on - Apple Support

If you can't pair an Apple Watch with your iPhone - Apple Support

Does your watch turn on, if you force restart it by pressing the Digital Crown and the Side button together for about 10sec, until the Apple Logo comes back again?

And, your Wi-Fi network has to have a 2.4GHz connection, since the Apple Watch SE 1st Gen can to use the 5Ghz network.

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copied from: If your Apple Watch shows an iPhone and a watch or a red “!” - Apple Support