Recently, we had someone to come by and replace a pipe in basement. He had to turn the main water valve to the whole house off. When he turned it back on, cold and hot water to the whole building (2 flat) are running very slow. How can I resolve this issue?:(

Perhaps he did not open the main valve completley.

Are you on well or city water.. And check HKstrouds question first.. this is very likely if you have a gate valve or similar.. in other words can seem open but will still open further.. turncounterclockwise until you feel handle seat... let us know.