The most likely cause of your missing email is that you have recently logged in with an email client (POP login) like; Microsoft Outlook, to check your emails, and it was not checked/ticked to leave the emails on the server.
All email programs have a setting to allow you to decide whether you wish to leave your messages on the server after you have downloaded them, and how long to leave them there.
If you wish to use POP login some of the time and Webmail or IMAP login some of the time, you should locate the setting (usually found on the advanced email settings page) in your email client software for ALL POP logins you have setup, and set it to leave your emails on the server for say 60 days.
The setting is usually called something like "leave a copy of messages on the server" or "keep messages on the server".