After I enter my username and password and click \"Login\" it sends me back to the Login page with NO error messages. The URL changes to include a session key and that\'s it
Clear the contents of the directory var/cache
and try again.
If that doesn't help check the table core_config_data
in the database for records with a path
value of "web/secure/base_url", "web/unsecure/base_url" and "admin/url/custom". These should match the page you are trying to log into (but without "index.php/admin/" at the end)
Finally check your browser's cookie settings and that the server's PHP is able to set cookies.
I just encountered this problem on one of my servers (not a localhost installation). Turns out the system account was over quota, so I presume Magento was unable to create the necessary temporary files. (I noticed this when trying to create a directory on the server over ssh.)
So... maybe check to make sure the account under which Magento is running isn't over quota on your server? At the very least, it might not be a Magento problem directly.