Start with quick checks: confirm your site address, database name, user, and...

https://atomic-wiki.win/index.php/What_Is_a_Costs_Agreement,_and_Should_I_Sign_It%3F

Start with quick checks: confirm your site address, database name, user, and password, then check whether your host is down. If details look right, review wp-config.php, database access, and server logs