Once you getting this type of error, you should have to mug up this error. There are issues with your database credentials. You have entered wrong database credentials in your wp-config file or your database (phpmyadmin) credentials are changed on server. It is also possible your database has been corrupted, but my experience is majorly database credentials are not matched with your wp-config file.
Note: Before you make any changes (either database or wp-config), you should have to take a backup first.
