Hi,
If you were able to get your watch to unpair from the watch app, but the watch itself has the white screen and/or did not unpair correctly, you will need to erase all content and settings of your watch and attempt the pairing process again.
To erase all content and settings, place the watch on the charger, press and hold both the digital crown and side button until the screen goes black. When the screen goes black, release the buttons, and then press and hold the side button until you see reset or erase. Confirm the erase. The watch should then restart on the pairing simulation.
HOWEVER, if the above does not apply to you, and/or your watch is running extremely outdated watchOS, and has no accidental damage, go ahead and contact Apple Support.