Access denied for user 'root'@'localhost' using Password NO
It's due to incorrect permissions on /etc/mysql/debian.cnf file.

Do a long-listing of that file first to check current permissions:

sudo ls -l /etc/mysql/debian.cnf


If group or other fields has write permission enabled, run:

sudo chmod 644 /etc/mysql/debian.cnf


and then restart mysql:

sudo /etc/init.d/mysql restart