There was a new version of Zephyr Squad that was released (8.0.1) over the weekend (10/9-10/10) which is what has caused this issue. We have already identified a root cause and will be deploying a fix so that it does not impact any more customers.
We had earlier (couple of weeks back) fixed an issue with some language settings that would check and automatically create TEST issue type for different languages. During 8.0.1 release since there were Atlasssian descriptor changes this effectively re-installed the application which triggered the language change to kick in and it has incorrectly modified the registered TEST issue type. This has not resulted in any data loss and all the data is still available.
If you/your instance has been impacted, please reach out to our support so that we can manually reset your TEST ID which should bring your tests back and should restore full functionality. We appreciate your patience and understanding. As always, if you have any questions or issues, please contact https://support.smartbear.com/zephyr-for-jira/