fdj12 "“folder empty”
If 用心棒 's Ctrl + Shift + R option does not help it would also be worth checking in each of the "Year" subfolders within the "Archive" folder in case they got moved into an earlier year sub folder.
It is also possible to move/archive emails to the main Archive folder itself rather than in to the individual year sub folders.
Also, I presume that you I also looked in the Trash Folder? In the "Move" menu the "Trash" option is just above the "Archive" option so you might have clicked on "Trash" rather than "Archive".
If all that still does not reveal the missing emails, then I'm stumped for the moment.
Coenoby