You may notice the following error when attempting to connect to a database:
SQLSTATE[HY000]  Access denied for user 'db_user'@'localhost' (using password: YES)
This error typically indicates that the credentials used for the database user are incorrect.
To determine if the credentials used are correct, please attempt to connect from the command line by typing the following:
mysql -u $db_user -p
Where $db_user is the actual database user.
If you are still unable to connect to MySQL with the specified database user, then chances are the password is incorrect. You can resolve the issue by resyncing the account's password. One way to reset the password is via WHM by going to:
Account Information » List Accounts
Once there, you will want to expand the information for the account by clicking on the plus sign.