why does all the electric appliances in a house stop working when the main switch is put off ????

A circuit needs to be closed in order for current to flow through it and thus all electric appliances to work. Switching off the main supply involves breaking the circuit and hence the current flow through the circuit (as mentioned by @Awani). This is why electric appliances (like fans, fridges etc.) do not work.

@Awani... Good answer keep it up!