Hi
I'm afraid that you cannot fix the issue yourself.
You will need to contact Apple Support (mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider to arrange for your Apple Watch to be serviced:
- US: Apple Watch Service Answer Center - Apple Support
- Elsewhere: Select your country for Apple Watch service and repair information
More information:
Get help if your Apple Watch displays a red "!" - Apple Support