How much damage is done if oil pan blows off and apart while driving down rd

If your oil pan literally came off your engine, you'll need to replace your engine. Oil pans do not come off easily and usually it's due to major engine damage. Seek professional local guidance. https://repairpal.com/repair-shops/buick-repair-and-service-in-denver-colorado

In this case it is a total loss - car is beyond reasonable repair.