Missing user on logon screen

One of my users has disappeared from the logon screen.
When I boot up, this user is automatically started.
If I do a "switch user", I cannot get back to the original
"missing" user until all other users log out. Then, by
pressing CNTL-ALT-DEL twice, I get a logon dialog box for
the "missing" user and can log back on.
When I bring up "User Accounts" this user is not listed.
However, if I try to "add" this user, the system says I
cannot because they already exist.
So, my "missing user exists, I can sort of use it, but I
can't get to it to even see what the settings are, and
possibly fix whatever got corrupted.
This machine is a week old, and worked fine until this
evening when this account went stealthy. My wife (the
missing user) thinks she is now an Orwellian "un-person".