Pull the cord dangling from your garage door opener to release the door. If it falls closed, there is something wrong with the opener tension.
Set up a ladder underneath the opener, and find the adjustment screw for the springs at the back of the unit. Turn the screw slightly clockwise to add tension to the springs.
Try opening the garage door again. Continue adjusting the screw until the door opens completely.
Inspect the track for obstructions. Have a friend hold the door up as you clear out any dirt or debris to prevent the door from falling on you.
Inspect the track for any bending. If the track is slightly bent and the roller cannot pass, carefully bend the metal back into place with a pair of locking pliers.
If the track is badly bent, you should replace the track.
Tighten the tension rods on the back of your garage door if the panels are starting to buckle by turning the large turnbuckle a few turns. Most garage doors have these rods installed; if yours doesn't, purchase them from your local hardware store.