The first thing i would try would be to take the aerators off the faucets and the shower heads off and flush the lines out by running the water at full blast. Probably got crud in the lines after sitting so long.
Did you drain the tank for the winter? if so double check that you have the incoming water valve wide open.
If there old gate type valves it's common to have the valve fail on the inside so the gate does open up, the hande will turn but the shaft snaps off inside.