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.