I changed the root directory's permissions to 664. Now my display and keyboard are disabled. How can I change it back ?
You could try to boot the system with a Live CD and see if you can change the permissions on the root directory by mounting the hardrive as a secondary device.
You'll need to know which were the permissions by default.
Try to boot from a Live Cd/DVD/USB.
When you are in mount your booting parition and then