iCloud email disappeared from Inbox

I have a problem with emails disappearing from the inbox. New messages arrive but nothing older than two days old remains. The sent messages folder is full; it is the inbox that has the problem. This is not a client side problem. I have logged into the iCloud account using Safari and see the same problem. This is the account of a friend who says she noticed it just a couple of days ago. Prior to that there was no problem. Emails do not appear in the inbox but can also not be returned using search; they seem to have disappeared entirely and are in neither the archive nor deleted items folders. This is causing a lot of stress so I would be very grateful for some suggested solutions.