Database connection error after upgrading WordPress

I encountered a weird database error after I upgraded my wordpress from version 3.5 to the latest version, I believe it is version 4.3. So after I updated my wordpress, my site start to shows error message “Access denied for user ‘user’@’localhost’ (using password: NO)” even though the database user has been granted all permission. I am also unable to access the wp-admin.

Is there anyway I could fixed this problem, or should I asked my server provider to re-roll the database.

Thanks in advance!