I assume config.php contains definitions for _HOST, _USERNAME, _PASSWORD, and _DATABASE and they haven't changed?
The error is very explanatory in that either the _DATABASE doesn't exist or has changed name or the _USERNAME can't be applied to it.
Also I hate people using select_db with mysqli.
You can change the two lines:
mysqli_connect(_HOST, _USERNAME, _PASSWORD) or die('Could not connect: ' . mysqli_error($link));
mysqli_select_db($link, _DATABASE) or die('Could not select database');
into
$link=mysqli_connect(_HOST, _USERNAME, _PASSWORD,_DATABASE) or die('Could not connect: ' . mysqli_error());
because mysqli_connect can take 4 parameters. Not a major thing (and not the cause of your bug) but just neater and quicker.