I had the same issue this morning. It turned out to be a problem with:
found a php issue in the php.ini that was causing the issue.
The guy that handles my server account was able to fix it. But it was the same exact problem you were having as well as customers with exisiting accounts could not log in a well.
This is ther error message at the top of the page when a user (customer) tries to login:
Warning: Variable passed to each() is not an array or object in /home/wireless/public_html/secure/includes/classes/db/mysql/query_factory.php on line 112
Warning: session_start() [ function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/wireless/public_html/secure/includes/classes/db/mysql/query_factory.php:112) in /home/wireless/public_html/secure/includes/functions/sessions.php on line 100
Warning: Cannot modify header information - headers already sent by (output started at /home/wireless/public_html/secure/includes/classes/db/mysql/query_factory.php:112) in /home/wireless/public_html/secure/includes/init_includes/init_templates.php on line 78