Apple watch flips to airplane mode on its own

Have you possibly put your iPhone into Airplane mode? If you do, the Watch will go there as well, and if you take the iPhone out of Airplane mode, the Watch will not go out on its own. You have to change manually.

Try this;

To force reset your Apple Watch, press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.