I beleive a blockage has occured in the valve , this could be lime scale from your hot water heater or regular debris present in piping ,water delivery systems. Id try turning off the water main, remove the valve, see if there are any visible debris in the valve seat, then turn on your main to flush out any anything and check the flow. You may need a rebuild kit, but probably not. It is normal for debris , sand , small gravel , limescale from hot water heater , to mess with plumbing , faucet screens and shower valves. And Shower Heads...I installed an inline whole house water filter on the main a few months ago , one with a clear casing , so you can see the filters condition. I was amazed in one week it was full of white crap and rust , the white crap looked like cut PVC debris. There is always Line Maintenance going on everywhere,, and any crap will get stuck if its to big to go through the Next Hole. Good Luck