Hi Lynda,
Normally the process should simply overwrite existing users and add new ones. If you used the option "DELETE all users on this list before importing", however, you may experience what you report. Could you have used this option and also clicked "OK" when it asked if you were sure you wanted to delete all users? If not, please try to recreate this with a test list so I can fix the problem.
You may want to contact your web host about obtaining a copy of your user database table (lm_users) from yesterday. They may not make a backup every day but it's worth a shot if you lost some good leads.
Regards