The navigation system in my car doesn't work.
The navigation system in your vehicle is a common failure. If this occurred right after a battery change then a code needs to be put in to get it going again. If the system is not turning on at all then I would typically check the fuse for the system first. If the fuse is fine, then the navigation unit has failed. This can go out all of a sudden when the internal hardware fails. If you need to have this looked at, consider YourMechanic, as a certified mechanic can come to your home or office to find out what the problem is.