Cold water not working following water supply (mains) failure

WIAWSNB

Sorted yet, Elika?
Almost certainly this will be an airlock in the cold pipe run from the CWS (cold water storage) tank in the loft to the tap and loo (and likely cold side of the shower) in the ensuite. If, during the 6 hours, you continued to use water (hot or cold) until the CWS effectively ran dry, then air would enter the supply pipe and settle in any 'high' spots in the run.
It might be that the cold pipe run from the tank to the ensuite is less well laid out than the others, say by having a horizontal section with a slightly raised point where air could settle and be reluctant to move. Or it goes up and around something at some point - that's where the air would settle.
You clear an airlock by attaching a hose to the offending tap's spout fed from the kitchen cold tap, which is supplied at mains pressure, and will be far more powerful than the CWS-fed taps. (Often the garden tap will be at mains pressure too, so can be used if easier).
The higher mains pressure will push the air bubble back up to the CWS, and the normal flow should resume once the hose is removed.
Some hints. Do this gently. Open the offending cold tap fully. Find a way of connecting the hose end to the spout - this can be as simple as holding it there and wrapping your hand around it, perhaps with a cloth, tho' expect side-spray! 
Shout to kitchen tap controller to slowly open the kitchen cold, and run it at, say, a medium flow. If the ensuite loo cistern is empty, expect this to refill.
Count to 20, remove the hose and point it into the sink (or shut it off if you have a nozzle on it), and observe the tap's flow. Has it been restored?
If no, reapply the hose, ask for it's flow to be increased a bit, give it another 20 seconds or more.

Elika0215

Thank you all for the messages. The hose from the kitchen (mains) to the ensuite tap was successful, so all working fine now.