First, let's define what a bad relationship is. It could be that what you look at as a bad relationship is just your teenagers wanting more space as they go through the process of separating from you while they learn who they are and what they want from life. The opinion of their friends is of the utmost importance, and their parents are relics from a century or two earlier, so to speak. You make an innocent suggestion, and your daughter blows up; you ask your son if he has enough money for the bus, and he rolls his eyes and tells you to get off his case. This doesn't necessarily signify a bad relationship; rather, it's how teenagers try to strike a balance between being their own person and living at home with the rest of the family.
On the other hand, if you were firmly in control and dominated your children as they were growing up, the teenage years could be the chance for your teens to pay you back in spades for whatever mistreatment (real or imagined) they suffered at your hands. It's pretty hard to force your teenagers to comply with parental demands, especially if they're bigger and stronger than you are. If you previously motivated them by fear and the fear is no longer there, what's left?
We hope that love and respect are still there. If your relationship hasn't exactly been characterized as warm and respectful until now, it's worth it to start working on a positive relationship. You don't want to turn your home life into a venue of daily power struggles and push your teen into the arm of the wrong friends. Professionals, neutral third parties and members of the clergy can help you and your teen reconnect; even if it entails some change on your part, you and your teen should be able to patch up your relationship.