If you just upgraded your Joomla 1.5 and you can’t log in to the Joomla 1.5 administrator area or User Login then I may be able to help you fix it. This “bug” seems to be present in nearly all Joomla 1.5 releases.
Fix is to enable Joomla Users in the Plugins database settings. You can do that using the following mysql query:
UPDATE `jos_plugins` SET `published` = ‘1’ WHERE `jos_plugins`.`name` =’User – Joomla!’;
You can run that via PHPMyAdmin or via your favorite mysql editor.
Let me know if it worked for you.