If you garage door opener is made after 1993, then you have safety sensors, or photo eyes, that help prevent the door from closing on anything valuable, like your car or person. The garage door opener will operate if you hold the wall button down until the garage door shuts completely. This is a bypass feature to get you through until a professional can come out and fix the issue. There are troubleshooting signs that will allow you to diagnose this issue before you call a garage door company to make an appointment for a repair.