It may be that the folder have been collapsed. Next to the main folder click on the triangle to expand that folder. If there are sub folders you need to do that as well. If you are in an office environment talk to your networks department to make sure there in nothing else going on due to a bad update or outside attack to the mail server.
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@JH1999, what problem you have? Is this related to Outlook? If yes, what version you are using now? What environment is there? Please describe more.