After logging into your cPanel account, you notice that one or more email accounts have gone missing.
When accessing the main Email account interface, the email account list is either incomplete or empty.
If the email account list is empty, or missing users, but you are sure they actually exist, look at the /home/$user/etc/shadow file. There may be some iteration of a backup, and if so, you can simply move this back to "shadow". If there is no backup, you will need to edit the shadow file and copy one of the entries to recreate the entries for your additional email accounts.
After this has been done, if the email accounts do not appear, you will need to move /home/$user/.cpanel/email_accounts.json.
Once that is done, reset the password for each of the users that were recreated.