It is the garage door springs that support an lifts the weight of your garage door.  Opening and closing your door creates pressure while tightening and loosening the springs at the same time. Eventually the steady constant use can cause your springs to break.  Over time the metal coils on springs will loose their ability to stretch and contract.

If your springs break while you are around there will be no mistaking what has happened.  They make allot of noise when they fail.  If your springs are not functioning the door will not raise and it may lower at an uncontrolled rate of speed.