In a dc circuit, if the wire of the negative terminal disconnected and grounded to the earth, what will happen then?

You will lose your power supply or simply the same as disconnecting the supply of your circuit. However, if your circuit is earth grounded nothing will happen. Some circuit or equipment uses earth ground together with DC supply ground. A common practise though is chassis ground is always connected to earth ground to prevent electrical shock. 

in dc circuit nothing will be happend

its simply cut of ur supply and crurrent will be grounded....and nothing will be happend.........