Put an new engine in and now it won't start what could it be?

Hello, wow, what a wide open question! Did you put the replacement engine in yourself? Start with the basic elements: Fuel, spark, air and exhaust. Ensure all is getting to the engine. If something is missing, focus on that component. It isn't terribly mysterious -- just have to look at the basics. Recommend a local shop be consulted for help: http://repairpal.com/jeep-within-10-miles-of-mohnton-pennsylvania-19540