It is very highly likely that you have a broken spring that should be replaced by an experienced and qualified garage door service technician. If your springs (torsion springs) are on a shaft across the top of the door, you will see an approx. 2" separation in the spring. If your springs (extension springs) stretch along the track on the sides you will find that they are very obviously in two separate pieces. It is recommended that you replace both springs if only one is broken since they both have the same wear and tear.