You can reset the password directly in the database. Find the table named CMS_User and clear the password field for appropriate user name row. Then you can log in to the CMS using blank password and change it in the Administration section. If you store passwords in PLAIN text and not in SHA1 format, you can see appropriate password in table above directly.
Second option how to retrieve a lost password is to use the forgotten password link. Please go to some page which requires logon and click on mentioned link, enter you e-mail address and the new password will be send to this address (if it exists for some user account).