The most common reason for the disappearance of mail from the mailbox is the connection of the mail client to this mailbox via the POP3 protocol, which, after downloading the mail, completely or partially deletes it from the server. You can check for POP3 connections in log of authorizations in the mailbox.
If since the disappearance of the letters has passed less than 1.5-2 hours, there is some chance to restore the contents of the mailbox from backup... In this case, you must immediately contact technical support, preliminary loggedin, and inform the name of the mailbox in which the letters disappeared, so that administrators can check the possibility of mail recovery.
If more time has passed since the mail disappeared, unfortunately it is impossible to restore it by means of hosting. In this case, the missing letters should be searched in mail clients in which a connection to a mailbox is configured.
To prevent the problem from recurring, you should: