Well, they're not entirely gone, because they're still there on the disk, but KMail is refusing to see them.

Been having GMail problems (as last topic) and, presumably separately, mail seems to be going into the inbox now, and then mostly not getting pushed through my (myriad) filters into the correct places.
Looking further, it seems all my accounts (they, at least, are all still present) have the inbox as the receiving folder. I'm *certain* I didn't set it like that. Accounts have corresponding folders.

I restarted the (Akonadi) server using the Akonadi Console, which now doesn't crash after stopping the server, which is good, however it was immediately after that that all my folders are gone.

I can't seem to re-point my accounts at the correct folders, the folder entry is only by selecting a folder from a tree, and the folders that used to be there are no longer there to be selected.

Is there a way out of this (while retaining KMail)?

"Is there a way out of this while retaining Kontact?"

It would appear not.
I've finally given up, after years of trying to ignore KMail's problems, enough is enough.
Evolution isn't so bad, and has lots of advantages.

Now to to try to ignore the knee-jerk reaction of throwing out any application beginning with K