I read all the answers, tried all the suggestions, had zero success
Can't believe Gmail is so hard to manage in this arena
I don't remember who sent me the messages I accidentally archieved
WHY is it so hard to find them?
It shouldn't be hard to find them. Even archived e-mails are in the All mail selection, see the picture below.
If you don't see them there, you probably deleted them by mistake (then you should try to check the trash).