What error are you getting when trying to access the ACP?
Have you tried using Recovery Mode?
Once you set your constants file, clear your browser cache (to make sure you don't have an active ACP session) or just use a different browser. Login to ACP. It will run through a process to disable anything in the ACP that might be messing things up. Once done, it will tell you to remove the constants.php variable. You should have access to ACP again and can re-enable plugins, themes, etc one at a time.