Error establishing a database connection

Hi
I’m hoping someone can help me with my problem.
I installed WordPress to a subfolder and now i move it to the root but it keeps giving me an error message ” Error establishing database connection” i checked the database in the config file and it’s ok. And i already changed the WordPress address URL and the site address url. I have tried to edit the URL in the database but it’s still the same error I don’t know where is the problem coming from
Please anyone has any ideas i just need to get it back to how it was??

Any one can help me solving this issue??