If you can login as root, then you can query the user table like so:

USE mysql;
SELECT user FROM user;

This should return all user names in the system. However, if you don’t know the password, you’ll need to assign it again as MySQL encrypts the passwords. So lets assume the database name you want your user to have access to is called mydb, then you would assign permissions like so (assumes your username is myUser and you are going to make calls from localhost…if making calls from another computer, replace localhost with that computer’s IP):

GRANT ALL ON mydb.* TO ‘myUser’@'localhost’ IDENTIFIED BY "yourPasswordHere";