Why is the circuit shorting after connecting a new light fixture?
Every white wire is not a neutral. If you see an "extra" white, you should not connect it to a bundle of whites (which are neutrals) in the box. The extra white must have been a hot -- either switched hot or an always hot. If you know which white was originally not connected, then disconnect it from the other whites. 

The previous guy probably also improperly connected a black to a group of other blacks. The proper connections might be to leave the white disconnected in the box with electrical tape over the end. Or the proper connection might be to connect this extra white to a certain one of the blacks. This might restore the circuit to functioning, but at this point it would be for the best to call an electrician.