Because our shower pipe is too low for my 6' tall husband, I installed the Waterpik Flexible Shower Head which attaches and then snakes up to whatever height you like. So, it is in an S shape.
To
I also removed the water regulator because we have low water pressure here, and the directions said we could remove this to increase flow.

I ran the shower for a while to be sure there were no leaks, and found none.

So, this morning my husband went to take a shower. He ran the water to get it hot, shampooed and rinsed, then before he could bathe, there was a horrible, loud sound in the wall, and the water from the shower head stopped. He pressed the button to put the water back to the tub, and the sound stopped and the water flowed just fine. Each time he tried to move it back to the shower, he got the sound and no water.

What on earth!

Your waterpik has screens, usually in two places. There is a screen right where you connected to the old shower head. Another screen is at the other end of the flex tube before the head. I think that some scale may have broken loose and plugged the screens. Try it and get back to us if this hasn't helped.