Make sure root pass is in /root/.my.cnf
This is usually the first root pass the box was given once cpanel is installed.
If it still does not work, you will need to reset the mysqld root pass.
First, stop the mysqld, usually
Then stop chkservd to keep it from interfering with mysqld while you work on it
Start up mysqld , but, without the grant tables,
mysqld --skip-grant-tables -u mysql &
Then change the pass..
mysql -u root mysql
UPDATE user SET Password=PASSWORD('new_password') WHERE user='root';
Now you just killall -9 mysqld and start it up normally with it’s safe_mysqld