The upper intake manifold may develop an internal coolant leak causing one or more cylinders to fill with coolant. This can cause the engine to "hydro-lock" and not turn over. Removing the spark plugs will allow the engine to turn over pushing the coolant out of the spark plug holes. Replacing the upper intake plenum and gasket is commonly required to correct this condition.
The engine can be very difficult to start after this problem has been repaired, it can be helpful to clear as much of the coolant from the intake manifold and cylinders as possible before reassembling the intake manifold.