If you set a login page to a page that doesn't have the login control you can do the following.

Update the portal table and set the LoginTabId column = null

Then save your web.config to trigger an application re-start as the tab settings are cached.

If you set an incorrect login page and it is on the default portal use this script.

UPDATE Portals
SET LoginTabId = null
WHERE PortalId = 0

Then add a space to your web.config (In a non critical area) and save it

You should now be able to login.

NOTE: If you have multiple portals be sure that you use the proper PortalId!!!