can’t log in to control panel

For some reason, a WordPress blog I work on all of a sudden no longer allows any of it’s users to log in, it just hangs on the log in page before timing out and giving a page not found error. Here’s what I’ve tried:

– upgraded to latest wordpress
– set up new database on server and connected to it -works fine (so it’s not the wordpress files or the server, must be a database problem I guess?)
– ran a repair on the database using phpmyadmin
– reset passwords on accounts (the password reset system still seems to work, although I get the same result when I actually try to log in)
– cleared cache, cookies, etc

Can anyone help?
Chris