My call logs are all accounted for. I also get a monthly spreadsheet report (you can make it a daily or weekly as well) emailed to me with my call logs for permanent storage. You can configure this on the Call Logs tab.
Are the call logs gone for just your extension or for all the extensions on your account? If you call into support (1-888-898-4591) we can try to get the logs back and viewable on the RingCentral service site.