Primary User missing after macOS Sierra Update
This can happen when some users are not FileVault Enabled. Log in with another user and then check out security system preferences.

Thanks for the help, @bret7600. FileVault pointed me in the right direction; the solution was the same as here User account doesn't appear on first login screen I also had to recreate the other account (Account B) which showed up on first login screen but not in User &amp; Groups to get everything back to normal.