If your vehicle has been towed it doesn't necessarily mean it's been impounded.
Denver, unlike many municipalities, doesn't automatically impound towed vehicles. Vehicles that are towed are often relocated to a nearby location and parked legally there. There is a $100 tow fee incurred for moving a vehicle.
If your vehicle's been towed, contact the Denver Police Non-Emergency line to locate it: (720) 913-2000 (when you get the recorded message, hit 0 for the operator, they will look up your vehicle's location)
If your vehicle has been impounded, call 720-865-0470 to retrieve it. More information on vehicle impound can be found at the Denver Sheriff's Vehicle Impound Facility.