First and foremost, a garage door, by design, contains springs designed to balance your door and make it easier to lift. Those springs are under incredible amounts of tension. If a spring breaks or is improperly released, it can cause incredible and potentially fatal injuries. Keep in mind, when working on a garage door spring, it is likely that your face and head will be close to it, meaning that your most sensitive area will be in the direct path of the released spring.
The power to your garage is out with the door shut, and you need to get your car out or you'll have to take a bus. You'll notice a cord—usually with a red handle—dangling down from the guide track that the opener uses to open and close the door. This is the manual override. Once pulled, it allows you to open and close the door with your own power in the form a little elbow grease.