The post title is misleading.....it implies the car won't start BECAUSE of the head gasket change. It seems the car overheated, died, and then refused to start. The head gasket change was done to fix the problem but did nothing to help the situation. There are a whole host of troubleshooting steps to perform....compression readings as suggested by 1970cs is certainly one.
It would help to know the engine and mileage of your Impala. Were there any other issues prior to the overheating? Trying to help, but there are more questions than answers at this point....