If your Apple Watch won’t pair with your new phone and is stuck on the black screen with the white Apple logo, simply follow the steps given below:
In order to restart your watch, hold the side button and the Digital Crown together and release them.
Now that your watch is restarting, push and hold either the Digital Crown or the screen firmly.
Finally, restart the device by pressing the Reset button.
You can pair your watch with your iPhone after it has reset following the instructions given under ‘How to Pair an Apple Watch with a New Phone’