You can't focus a disabled element. Remove the disabled attribute first.
$('body').on('click', '#button', function() {
$('input').removeAttr("disabled");
if($('input[name=textfield-contact-name]').val() == '') {
$('input[name=textfield-contact-name]').focus();
}
});
Plus, I think the markup for disabled should be:
<input type="text" name="textfield-contact-name" disabled="disabled">
I also changed your jQuery selectors ( removed the " ) (that's the way it worked for me)