Hi @wi77iam Welcome to the Microsoft Q&A platform, thanks for posting the question and for using Azure Services.
If the server is configured for CMK (Byok) and the server is inaccessible, then it means the access to the key vault is lost. I would check the CMK \AKV configuration and revalidate it to make sure the server is back to a ready state.
Please let me know if this helps
Regards
Geetha