Sometimes these springs break, and unless you really know what you are doing, call a service and have them replaced; do not replace them your self. The tensions on the springs are very high, and it could be very dangerous if you attempt to change them yourself without prior knowledge.
Most residential garage doors are built from single horizontal panels that are joined together with hinges as the door travels up and down on steel rollers that run on a track on each side of the door. The springs help the door travel up the roller by their contraction action, so the motor has less stress as it operates the door.
Typical springs are guaranteed for a 5 to 7 year period, which of course can vary depending upon the use of the door, how many times it is activated and so forth. Springs that have to operate in very humid climates will not last as long because of corrosion and cracking.
The door maintenance will have an effect on the springs as well. The bolts on the hinges must be maintained and tightened on a regular basis and the tracks must be maintained and kept from coming loose as well.
Additional weight on the doors by adding glass, for example, or even additional coats of paint will affect the wear of the springs. If a spring breaks, the garage door will fall, and a free falling garage door can be very dangerous. If you have a broken spring, you should call a garage door repair service, because repairing them your self can be difficult and dangerous, unless you have experience in that endeavor.