Many years ago I thought my water heater was clogged.
I was ready to replace it when I removed the flexible copper pipe off the outlet. That flex pipe was about 75% clogged at the fitting with scale.
I replaced that copper flex pipe and solved the issue for $10.
This may have nothing to do with your situation, but in the slight chance it does hopefully it will help.
It looked like one of these:
EDIT: if the upstairs shower flows no water, hot or cold. I suspect your pressure regulator is shot.
Go to the hardware store and get a $10 hose bib pressure gauge and see what the pressure is. i bet it's under 10 psig.
