sliding glass doors have a knock-out for a key-lock. If the key-lock isn't there, and all you see is a dime-sized indentation, take a screw diver and a hammer and knock out the metal circle. You can now access the opening mechanism with a turn of your screwdiver... then buy a lock.
If they have an electronic garage door, you can use a coat hanger to grab the dis-engage cord from the chain... but that takes skill, a step stool, and a slight bending of the top of the door.
You can also call a locksmith.
mike, September 2007