Garage door springs can break for a variety of reasons. Extreme heat or cold, for instance, can stress the springs to the point of breaking, or it could be that normal wear on older springs could cause them to break. Whatever the situation, a broken spring can render your garage door inoperable. Your door will not be able to open or close when a spring is broken, and you should stand clear of your door until a professional can assess the situation and correct it. http://youtube.com/watch?feature=youtu.be&v=Z_eZc-kh40c
Guest 9232954 -- check out the other answers for this question, and the links below the answers for even more - seems from $125-400 range from different contributors (should be replaced in pairs for balanced lifting unless one spring is quite new), about $200-250 or 300 seems a common charge. The springs themselves generally run about $20-50 each depending on lead they have to carry.
10.8 After the cone reaches the mark, continue pulling the bar off the garage door and back toward the middle of the door with your left hand. Tighten both set screws 1/2 to 3/4 turns each after each screw first meets the shaft. This is the point at which you will feel the screw meeting resistance. If the garage door spring snakes after tightening the set screws, repeat the process and stretch the spring half as much as before. Residential torsion springs are stretched 1/8" to 1/4". Larger commercial and industrial overhead garage door torsion springs are stretched as much as an inch.

4.4 Take the spring on the left and place it at the left end of the door as pictured here. Notice that the end of the wire points to the right toward the center of the door. This is a right wind torsion spring. It will go above the garage door on the left side of the spring anchor bracket. The winding cone at the other end of this spring is usually painted red.
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