If you can see no external leak, then suspect an internal leak as in a Blown Head Gasket. This is very serious and severe engine damage will occur if you continue to drive the car. Get this repaired ASAP, and stop driving the car.
You do not let us know if you see coolant on the ground or on the engine. It's hard to diagnose with out a little more info.
A leak. You need to see where it is coming from.
You have a coolant leak.You have a coolant leak.
Year, make, model and engine info would help but, the intake manifold gaskets are known to leak coolant on many Chevy/GMC engines.
In general, yes a coolant leak in a 2002 Bonneville can be fixed. The issue whether it is worth the money to fix the leak. If the coolant is leaking from an upper or lower coolant hose, the repair is simple and inexpensive. If the coolant leak is from the radiator, it will be a bit more difficult and/or expensive to repair. Moving up the scale, if the coolant leak is coming from a manifold gasket, it will be still more expensive. Likewise for a coolant leak from the heater core. And at the upper end of the scale, if you are leaking coolant from a cracked engine block, you're looking at replacing the engine.
why does the coolant only leak when engine is off on my 1999 grand am?
This depends on where the leak is. A hose could be loose or broken or there could be a radiator or engine leak. The area of the leak needs to be found to be able to diagnose.
The leak could come from: hoses, punctured radiator, split gasket...you can put tracer dye in the coolant and search for the leak with a black light after you clean the engine.
If you have coolant on the top of the engine it could be a leaky thermostat housing or a leaking intake gasket.
If the smoke is coming from the exhaust then it's coolant. If its not turning over then it could be hydrolocked from a coolant leak.
It's an engine, not a motor, and a number of things. If it's a leak which allows coolant into the engine, it could get into the combustion chamber. This could eventually cause detonation, which could scar and damage the pistons. The vehicle could overheat, as well.
The water could be coming from a coolant leak higher than the plugs on the engine. Look around the head for signs of any leak as it is possible that there is a head gasket leak.
It could be a head gasket that needs replaced or a freeze plug needs replaced depending where the leak is coming from. It could also be a radiator hose.