When you changed over your new showerhead, did you turn off any valves anywhere in your house to stop the water? Did you turn off the water at your water heater?
Are you saying that when you turn on the cold water, everything is fine. But if you turn off the cold water and turn on the hot water, nothing comes out?
Descriptions of a young DIYer that wants to learn how to do everything himself. This is the kind of thing you don't see on HGTV.