Login page suddenly not working - can't log in to add/edit

[brian]

System configuration

CMS 1.4.1
php 5.1.6-20.el5_2.1
mysql 5.0.45-7el5
kernel 2.18-92.1.13.el5 (CentOS5)

This is a CMS that has been running since this spring.  All of a sudden (well, 4 weeks ago), the database suddenly stopped allowing logins.  I've gone into mysql, checked the database, changed all the passwords in the cms_users table using the mysql commands; ran mysqlcheck and myisamchk on the database, and even looked at the login.php script.  It appears to be the default cms made simple login script.

When I look at the adminlog table, it shows a lot of admin logins since this trouble started, so the cms made simple site is attaching to the database.  Am I missing something that should be happening on the back end here?