I seem to have accidentally deleted my first answer :(. You could try following [http://www.makeuseof.com/tag/two-ways-to-reformat-your-chromebook-powerwash-full-recovery/|this] guide to "power-wash" your device externally to the logon screen. I am assuming that you are using the device for personal reasons and will not need to "enrol" your device in to another organisation.
Regards
Alex