In my house the downstairs cold water comes directly from the mains.
Our upstairs cold water comes from a tank in the loft - there are often two of them - a big one supplying taps, hot water tank and toilet and a smaller one for the radiators.
The inlet to these tanks is a ball valve - yours may just be jammed but if it has been very cold where you live it may be frozen and the thaw may cause water damage from a split pipe.
Make sure there is no insulation underneath your tanks so the house heat keeps them above freezing. Insulate by making a big igloo of fibreglass over the top of them.
Good luck...
Peccavi
December 2009
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