This is also why I suggested disabling all 3rd party resources as the first step above.... 🙂
You might try using Recovery Mode to disable everything now that you are in this situation.
A few notes...
If you don't have a constants.php file, you're going to be creating one. Just paste the code it gave in.
When you add the constants.php file, clear your browser cache... THEN login to your ACP. You should get a message that resources are being disabled.
Once the resources are disabled, you'll need to turn off recovery mode (remove it from your existing constants.php file, OR if that was the only entry in it just delete the constants.php file). You won't be able to do anything in ACP until it's turned back off again after things are disabled.