Mail: iCloud deleted all mail accounts

When I turned on of my apple computers, a message from iCloud informed me that another device had been added (a new iPad) and requested confirmation. When Mail was opened, ALL the other mail accounts and e-mail address folders had been deleted!

I did nothing to delete them: I neither deleted mailboxes or purged mail. I had synced mailboxes in the past for years successfully, and enjoyed the convenience of all my mail accounts and keychains working together - now have experienced sabotage, Very strange - a spontaneous wiping out with no action of mine.

Several people have told me this happened when they updated to the new El Capitan operating system (and had to go to the Apple Store to have geniuses help them get stuff back, so I have NOT done this until this bug has been fixed.

How can they be restored, and how do I keep this from happening again?

More importantly, what is the mechanism that did this? I feel violated.