Powering down the computer during an update when it is already stuck rarely breaks it. Chances are high you'll get away with it and it will recover gracefully as it is meant to. Especially on Windows 10 and to a certain extent Windows 8.1, it does this fantastically effectively - it's getting really, really rare to see it fail to recover. Windows Vista and Windows 7... less so, but you'll usually get away with it :)