I have a friend in Cuba that has sent me numerous innocent emails over the years, they have all suddenly disappeared for some reason has anyone else had this problem?
Could be you might have then “inadvertently moved those emails to another folder” within your account or another mail accounts.
So suggest to “go to the search field at the top centre or top right” depending on your eM Client version, and select “All Folders” in dropdown field and then type a search for text that you know was in the Subject or Body of those emails and see if any of those emails then appear.
“eM Client Search documentation”
https://www.emclient.com/webdocumentation/en/10.0/emclient/default.htm#Getting%20Started/Search.htm
Now if the All Folders search doesn’t find any of those messages, then I would suggest as @skybat already advised above to “try restoring an eM Client dated backup” when you know those emails were there.
So If they were in eg: “Local Folder messages” and they are not in your current eM Client, then the restore is the only way to get them back.
Note: Before restoring an eM Client backup, make a new manual backup of your current database via “Menu / Backup” incase you need to revert back for any reason. You can see when the backup is complete in Show Operations via clicking the drop-down on the right of Refresh top left.
You can also do a eg: Partial Restore from an old eM Client dated backup or hidden database folder as per my post in the following thread.
See also eM Client official blog on restoring backups including Partial Restore backups.