Will not start after replacing battery

My vehicle had no problems and was sitting idle for six months since it became our third vehicle. Vehicle would not start even after jump starting. Replaced six year old battery and negative terminal connector due to battery acid. Vehicle was able to start but would stall if I did not press the gas. After continuing to do so, I heard a pop. Now the engine will not "turn" when turning the key. What may be the problem? What do I do?