2000 Chevy Monte Carlo Wont start after the cylinder head r

Engine Mechanical problem
2000 Chevy Monte Carlo 6 cyl 141000 miles

Hi, I really hope you guys can help me i've been stuck with this situation for about a week now. I had my cylinder heads replaced (2000 Chev. Monte Carlo 3.4 engine) along with spark plugs, spark plug wires and oil change all at the same time. Upon completion of the project, when starting the car, the engine would not turn over (it acts as if it wants to but its not getting enough charge; it makes a noise similar to when a car is out of gas). Upon reveiw of the situation the mechanic determined that it was the systems computer not sending the signal to the spark plug area ( even though there had not been an issue with the engine's computer system prior) and in turn not allowing the car to start. My question to you is 1.) Is the mechanic correct and what further actions should I take or, 2.) Is there something that we missed or that needs to be repalced for the car to actually start. Thank you for taking my question and God Bless!