iMessage conversations changed to Bonjour; how do I change them back?
In addition to unselecting Bonjour, which I agree was insufficient, I found that under Messages > Preferences, my Apple ID had been deselected.  Once I re-selected it, I could then identify the phone numbers and email addresses I wanted to use with messaging.

I suspect my original cause was my daughter's wanting me to turn off messaging for her email address (she is young and so her stuff is under my Apple ID) and somehow that turned off the MacBook's messaging entirely, not just her address.  Curiously, my iPhone was affected.

I also suspect I selected Bonjour in a futile attempt to fix messaging on the MacBook.  Hope this is helpful.