The simple way is to log

greg — Thu, 11 Jun 2009 13:47:00 +0000

The simple way is to log into your web hosting control panel (plesk, cpanel etc) and check the FTP users and passwords that are listed.
Some don't actually show the passwords, so perhaps try changing the password for a FTP user and then change it in your FTP client to the new one.

Also check in your FTP client that all the other details are correct - host/url/ftp account, username, password, etc. All of these details are generally listed in your server control panel.