The first suggestion before you take any drastic measures is to take the shower head off and see if any water comes out without the head on. If it does, something has plugged the head up.
Even if it still does not, I would feed a wire into the pipe and see if I can reach the valve before I start cutting walls in a shower.
And last but not least: the ball valve only has one particular position in which it works properly. And it has to be the exact ball for this model of the faucet. I don't suppose you accidentally bought a wrong ball and forced it in by any chance? I also hope someone didn't yank that handle really hard, way out of position so the ball was (and perhaps still is) facing a wrong way? Can happen if the ball slices off the key which in your faucet could be made of plastic and is now gone. Identifying the faucet and getting its parts diagram and installation instructions would be essential in this case.Â