What could be preventing the garage door from shutting

There are several things that could cause a door opener to not work properly. Tester101 put most of them in his comment, but most of those would at least result in the opener visibly and audibly trying to open the door, and then giving up.

This may sound counterintuitive but my first priority would be getting the light working. Most garage door openers use the light as the primary indicator of a problem, by blinking it a certain number of times. Replace all bulbs in the unit (preferably with bulbs designed for use in high-vibration applications) and see if the light will turn on.

The second thing to do is verify that whatever control you are using to try to open the door is properly connected and/or programmed. Replace any batteries, check the inputs on any hard-wired switch or panel, and have the opener "re-learn" any wireless control modules you're using. Understand that many systems require at least one hard-wired multi-function switch in order to access all the features of the opener.

Also check all the safety interlocks. Most openers on the market will not operate at all without the electronic eye system properly installed, connected, aligned and the lenses cleaned. Certain systems may have eyes at multiple levels to allow the eye to "see" things that could cross the threshold at different levels, like a car bumper vs a truck bumper; every installed eye system, and there must be at least one, must be working properly.

Next, pull the red handle on the traveller shuttle to disconnect the door from the drive mechanism. Press the garage door button, and see if the chain drive operates. If so, the problem is the door; it's locked, jammed or improperly counterbalanced. If it still doesn't move, the problem is the drive mechanism; either the travel settings are messed up, such that the door thinks it's fully up or down no matter what position it's really in, or else the motor has burned out, seized up, slipped a gear, etc. Try adjusting the travel and see if the drive will move. Also, some openers are pressure-sensitive and will abort closing if they sense that the door is pressing on something (like a kid who tried to slip through and got caught); that pressure sensitivity is often adjustable, and you may have it set to be too sensitive so it's tripping based on the weight of the door itself.

If none of these at least indicates a further course of action, you're probably looking at replacing the door opener.

It sounds like the controller needs to be reset and the remotes reprogrammed. Usually this requires physical access to the opener, but if you have a hardwired remote it might be possible to pair it via the keypad. Check the manual for your opener.

But back to the problem of how to open it. If the garage does not have an alternative access there should be a manual cable release which disconnects the door from the opener chain so that you can manually open it. Often this is in the form of a small key near your garage door. With the key you can open the lock and pull the manual release.

_{(source: garage-door-automation.co.uk)}

If the installation was done correctly, you have one of these you just need to find it. If you don't have the key a locksmith should be able to open it for you.

If you do not have a manual release then I think you will have to resort to breaking a window.