Overhead garage doors are massive beasts that sometimes don't behave like they should.  Add in a garage door opener and there are all sorts of things that can break or not work properly.  Here are some of the most common things to troubleshoot when repairing a garage door.

How to Repair a Garage Door Openerphoto courtesy of Marya flickr.com/photos/emdot/47650844/Garage Door Won't Close

Remote
If your garage door opener remote won't work but your wall mounted garage door opener does, you most likely need new batteries.

Motion Sensors
If you have a garage door opener, unless it is an older model, there should be motion sensors towards the ground on either side of the opening on the inside of your garage.  The motion sensors open the garage door when they sense movement so the garage does not close on an unsuspecting person.  If the sensors are blocked or misaligned, the garage door might not close at all or will open up after closing partially.  Adjust your sensors and make sure there is nothing in the way and try to close the garage door again. 

Garage Door Won't Open or Close Completely
Your garage door opener has limit switches that tell it how much to pull up and let down.  Your limit switches may need adjusting on occasion if they get confused.  Research your make and model of garage door opener and follow the manufacturer's instructions for adjusting the limit switches.  They are most likely located on the back of your garage door opener motor housing.

Garage Door Reverses Once Closed
Once again, the limit switches are probably to blame if your garage door opens again after closing.  Adjust them and try again.  If you don't have time to adjust them immediately, simply press the garage door button when the garage door is closed to stop the garage door from opening up again.

Release Cord is Pulled
If your power went out and you pulled the release cord to manually operate your garage door, you will need to engage your release cord in order for the garage door opener to work properly.   Some garage door openers will engage the release cord simply by pressing the garage door button.  Other garage door openers require your door to be in the closed position then pull the release cord towards the garage door motor until you hear a click (you may need to pull with a good bit of force).  Press the garage door button and it should engage. 

Warning
Do not try to fix or adjust the torsion spring of a garage door opener.  The torsion spring is under extreme tension and serious life threatening injuries can occur if not handled properly.  Hire a garage door repair professional if unsure of any repair procedure.