If a more complex issue is the cause, however, then a professional locksmith will have to examine the garage door equipment to figure out what's going on. Here's a list of what you can do to troubleshoot your garage door.
Check the Remote Batteries
The first thing you should do when the garage door isn't opening is check the batteries in the remote. This seems obvious but many people forget to try different batteries. If the old battery works after being rolled, it's a sign it's weak and needs to be changed.
Try the Transmitter
Another possible reason your garage door isn't working is dead transmitter batteries. Try opening the garage door with the transmitter, if it works, then you know the transmitter isn't the problem. If it still doesn't work, you can try changing the batteries.
Clear Objects Out of the Way of the Sensors
After ruling out the remote as the source of your garage door problem, check the sensors inside the garage. They are usually located about a foot off of the floor. One side has a green light and the other a red light. If something interferes with the beam sent across between the two sensors, the garage door won't open.
Check if the Sensors Are in Alignment
If the garage door still doesn't open after ensuring the path between the sensors is clear, they may be out of alignment. The sensors can become unaligned when bumped. You can ask a locksmith to check your sensors if you're unable to tell if they're the reason your garage door isn't opening.
How to Troubleshoot Garage Door Openers, ehow.com