ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)

after typing :

mysql -u root

I don’t know why ? but I know how to solve it.
First of all let’s disable password authentication :
Stop your MySQL daemon:

/etc/init.d/mysql stop

or

/etc/init.d/mysqld stop

Then run the following command line in background :

mysqld_safe --skip-grant-tables &

Secondly run :

mysql -u root

Finally, In your MySQL command line prompt issue the following command:

USE mysql;
UPDATE user SET password=PASSWORD("my_password") WHERE user="root";
FLUSH PRIVILEGES;
EXIT

Password is now reset to my_password. So just restart your MySQL server using :

/etc/init.d/mysql restart

or

/etc/init.d/mysqld restart

and use your new password to authenticate :

mysql -u root -p

All that remains is to enter your password.