loud bang when turned key

I think you had a small explosion in the intake manifold, or in the crankcase, either due to a sticking intake valve or ignition problem causing flame propagation through the PCV system. I recommend you get the car to a GM specialist for inspection and diagnosis. You don't mention whether the car runs or how well so I can't tell you more than that. If you want to find a specialist, here's a directory link for you: https://repairpal.com/buick-repair-in-jasper-florida