Hello Kramaika,
It sounds like your cellular plan was disabled on your Apple Watch. We understand how important this is to you and we are here to help.
Let's review the recommendations outlined here: Use Apple Watch with a cellular network
Specifically the section titled "Turn cellular off or on"
If the issue persists, attempt to remove your cellular plan completely and set it up again using this article: Set up cellular on Apple Watch
If the issue persists, can you provide a few more details? Which iOS and watchOS version are you currently using? Also, have you already spoke with your carrier?
Cheers!