Your cold water supply is off. All others upstairs etc. will be from the tank in the loft which will have water in it. Hot water from the hot water tank. Check your stopcock and if thats ok, phone the water company. There may be a burst pipe somewhere in the street. HTH.
Could be a drop in water pressure somewhere on the mains or air trapped in the pipes afterwards. Downstairs taps are fed direct while others route via your storage tank which may well not be being refilled as you use them ie flush loo, take a shower.
Your megaflow tank should not make a difference, it will get water from the main and be full so you can still use it until the water runs out. I think its unlikely you would get such instant limescale build up but we are in Scotland so I have no experience of limescale. Did you manage to find out what was causing it? Has it been resolved?