Usually its a tank with one or two pipe's coming out of it at the bottom.
If the tank is quite big and looks as though it would hold about 270 litres (60 gallons) of water. Then the chances are it supplies both the cylinder for hot water and the cold to the bath, basin and wc. The electric shower is probably off the cold mains water, which is a separate cold water supply to those appliances fed by the tank.
If you haven't touched any valves on the tank, what has probably happened is that when you turned the cold off that feeds the tank you have drained all the water out of it and air has gone down the supply pipes. When you have turned the cold back on its trapped the air down the pipe and made an air lock somewhere.
Try blowing down the cold tap on the bath or basin as though your blowing a trumpet, make sure the ball valve in the cistern is held closed and the basin cold tap is shut off. That may clear the air. Sometimes if the taps have nrv's or loose jumpers on you can't do any of these tricks.
You could try linking a cold tap to a hot one and see if that does it, it does sometime, but the pressures are equal so it might not.
If you get desperate, try somehow linking the cold on the bath or basin to the mains water supply and blowing it out with the mains water.
I've even cleared them by turning on all the cold taps and getting a short piece of hose and putting it down the cold water outlet inside the tank. Then giving it a good blow whiles not letting any blown air escape back into the tank.
I have even stuck a cork in the tank outlet and taken the valve head off and and blow from there whilst somebody held the cork in place.
Even sucking the cold tap sometimes works.
Even disconnecting the ball valve supply and fitting a hose on that then pushing it down the cold outlet and then turning the ball valve supply back on while holding the hose tight in the outlet.
It all depends on how hard its air locked.
What your trying to do is remove air out of a pipe, so any ideas of your own along those lines may help