So there's a nut inside that the handle screw threads through called a bonnet packing nut and if thats rusted inside sometimes doin what I told you will get the nut to sit flush again. Did it seem rusty, feel crunchy?, when you turned the screw? Its probably the gate valve inside. You can try a thin wire brush up through the tap, opening and closing it as u brush back and forth but usually with a gate valve if that first trick doesn't fix it, it needs replacing. Most people don't even try the trick i suggested cuz on the majority of em if u turn the screw too far the nut falls off inside and then the handle is useless.