Well, most Hardy Hybiscus wont reach a height of 6 ft. you most likely have the non hardy variety, which is a tropical plant and cannot be exposed to cold weather for long periods of time without significant damage. This will kill the entire plant, roots and all.
Hardy hybiscus die back in the winter, their roots survive which then send up new shoots in the spring, so hardy hybiscus will also take damage from the cold, but they come back. You can cut back hardy hybiscus in the fall to 2 or 3 inches from the ground, then cover it with mulch to help protect the roots.
So even if your plant is hardy, or tropical, cold weather will affect it in some way. Best thing you can do if your unsure of what it is, is keep the plant inside your home in a warm, sunny location.
Here are some pictures of both Hardy, and tropical Hybiscus
Tropical:notice the waxy leaves