Probably start by opening the supply lines just after the pump to see if water pressure is there, if so, reattach lines then open lines close to the zones to check for water pressure. If no pressure supply lines may be frozen. If you have pressure reattach lines then open return line close to zones to check for water return water. If no water your zones may be frozen. With out being on site this would be my process of elimination. Check all check valves that's on that side of the system.