If you have copper pipe don't just crank down on it you may twist the pipe in the wall. If you have to remove the valve you may need to open the wall back beyond the valve so someone can get a wrench on the pipe so the valve can be
loosened. I hope if this is the case that at least one side of the interior wall is where you can get to it. Been there done that.
"Forget the flat stuff slap something on the spinny thing and lets go, we're burning daylight" Bart Leetch
"If it ain't round you may be a knuckle dragger""Turners drag their nuckles too, they just do it at a higher RPM"Bart