Probably not only CalDav users but all user services right? The error message says your LDAP server can't be found.
Directory Service Problems: If this CalDav server is also the Open Directory server hosting the user accounts, check the Open Directory service for problems. If you are bound to AD, check that server to see if AD is OK and that server is on the network.
DNS Problems: Check your DNS records for the LDAP server- is it listed? On your CalDAV server, open Terminal and use nslookup to see if you can find the LDAP server. If using AD, is it possible that there are some "addomain.extension" DNS records referring to domain controllers that no longer exist?
Directory Utility: On the CalDav server, go to SysPrefs--> Users and Groups to see if the server you are bound to is green. You may need to launch the Directory Utility and verify your binding or even re-bind to the LDAP server.