If you have an indirect plumbing system, cold water that fills the toilet cistern (and possibly also bathroom taps) comes from the tank in the attic. Look for a quadrant valve (ball valve) in the water line that feeds these appliances. Sometimes this is located in the hot press near your hot water tank. These valves have a lever that should be in line with the water pipe when turned on, and perpendicular to the pipe when turned off. It is possible to inadvertently knock off the valve if you lean on it by accident. Before you do this, however, you need to confirm that it is not a gas valve which has been turned off for some reason.
Another scenario for having no water is that the ballcock in the cold water tank in the loft has stuck and the tank has emptied.