Thank you for posting! For the cloud edition of TestRail, we take nightly backups and can restore to any day’s backup as needed. We can restore to the night before the test cases were mistakenly deleted, but this would be a full point-in-time restore. Please note that this would reset all the data in TestRail, including all projects, all test results and any other contributions and changes your team has made since that backup. If you would like to restore a backup of your TestRail instance, please send the URL for your TestRail instance to the support team at email@example.com.
It is also important to note that any user that is assigned a role with permission to delete test cases and sections would be able to delete a test case. We generally recommend using the user roles and permissions to restrict and limit access to important delete functions in TestRail to a small number of users to prevent mistakes. You can manage your users and their assigned roles under Administration > Users & Roles. TestRail does not currently have any specific audit logs to determine which user deleted the test cases, however we do have plans to add more detailed audit logging in a future update to TestRail and I am happy to add your vote to this feature request.
I hope this helps!