Reset password

You can reset the root password by running the server with --skip-grant-tables and logging in without a password by running the following as root (or with sudo):

# service mysql stop
# mysqld_safe --skip-grant-tables &
$ mysql -u root

mysql> use mysql;
mysql> update user set authentication_string=PASSWORD("YOUR-NEW-ROOT-PASSWORD") where User='root';
mysql> flush privileges;
mysql> quit

# service mysql stop
# service mysql start
$ mysql -u root -p

Now you should be able to login as root with your new password.
It is also possible to find the query that reset the password in /home/$USER/.mysql_history or /root/.mysql_history of the user who reset the password, but the above will always work.
Note: prior to MySQL 5.7 the column was called password instead of authentication_string. Replace the line above with

mysql> update user set password=PASSWORD("YOUR-NEW-ROOT-PASSWORD") where User='root';

--
Chus ys

mkdir -p /var/run/mysqld
chown mysql:mysql /var/run/mysqld