Sometimes the driving force behind your garage door opener that chain drive- will require maintenance. This doesn't necessarily imply that something's dreadfully wrong with the door, just that the chian has perhaps stretched a bit, or some of the nuts or bolts have shifted somewhat in the natural process of time.
All you need to fix your own garage door chain drive is open-ended spanners that will fit the lock nuts mentioned above. Be sure it's a good fit (or an adjustable spanner) if you don't want to accidentally 'strip' your lock nuts and make them cylinders.
First, put the garage door in its lowered position, to take all of the pressure off of the chain in question. Facing the motor from the door, you ought to be able to see a threaded rod- this is responsible for adjusting the chain length on the right side of the door. All you need to do is loosen the lock nuts involved with the threaded rod, turning them in opposite directions and moving them about a quarter inch. That way, they won't lock up when you try to twist the rod.
Then go ahead and twist the threaded rod until the chain picks up all the slack and there is tension again. It should not be able to move outward more than about an inch. Re-tighten all of the lock nuts, and you're good to go!
Be sure to raise and lower any and all garage doors after doing any minor repair- just to double-check that there are no problems. And if you run into any issues, give your local garage door installation specialist a ring- they'll be more than happy to help you.