If the door has one of those infared emergency stops on each side of the door about knee high or lower, one of them may have gotten bumped and they need to be repointed at each other.
On the back of the unit there is a screw to increase the amount of force to close it. Try increasing that. There is another adjusting screw also.
If all else fails, pull the red handle to unlatch the door and close it manually. Post back if you fix it and what the problem was. Some of the roller bearings could have gone bad too.
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