What Stops The Door From Closing?
If something is in the way of your garage door as it closes, there should be a safety feature to stop it before it hits. The first thing you should check is the sensor on the opener itself. It's usually red and should have a small light/button next to it. Make sure this isn't pressed to the down position.
If that doesn't work, check for a second sensor on the door itself labeled "photoelectric" or something similar. The two should be far enough apart so that you can fit a finger between them easily, but not much farther. This one is normally white and sits inside the glass at the top of the door. This should also have a button located near it that turns it on or off. Make sure to double-check that this sensor is facing out towards where the opener will be when the door is closed, not inwards to get in its way.