If an email client, or a webmail client, is no longer displaying a folder in an IMAP-controlled mailbox, it could be because that folder is marked as "unsubscribed".

With IMAP, as part of the mailbox, a folder can be "subscribed" (default) or "unsubscribed". If an email client marks a folder as "unsubscribed" (which could be accidental), then that setting will be stored on the server, and the folder will no longer display/synchronise/search on any email client or webmail access.

To resubscribe a folder, login to webmail via yourdomain.com.au/webmail, choose Roundcube, then go to Settings > Folders, and check the box beside any folder that is unchecked.