auto_increment not working

I have a MySQL database with the following structure. The id_number field is a primary key and is set to auto_increment. But, auto_increment has stopped working, and now I have multiple customer records with a null id_number value. Anyone know why this would happen? TABLE STRUCTURE: +------------+----------------------+------+-----+---------+---------------- + | Field | Type | Null | Key | Default | Extra | +------------+----------------------+------+-----+---------+---------------- + | id_number | int(11) | | PRI | 0 | auto_increment | | enter_date | date | YES | | NULL | | | fname | varchar(35) | YES | | NULL | | | lname | varchar(35) | YES | | NULL | | | email | varchar(100) | YES | | NULL | | | address | varchar(70) | YES | | NULL | | | city | varchar(35) | YES | | NULL | | | state | char(2) | YES | | NULL | | | zip | varchar(9) | YES | | NULL | | | country | varchar(50) | YES | | NULL | | | phone | varchar(20) | YES | | NULL | | | browser | varchar(70) | YES | | NULL | | | comments | blob | YES | | NULL | | | registered | enum('true','false') | YES | | NULL | | +------------+----------------------+------+-----+---------+---------------- + -- Randy Perry rgp systems Mac Consulting/Sales