You must hire a professional garage door repair technician right away. The loud bang is most likely caused by a snapped garage door spring. Don’t try to repair or replace the spring on your own. More importantly, you must not disconnect the springs yourself.
Here are additional precautionary measures you must take:
• Do not leave the door open because someone may attempt to close the door without knowing that the spring has snapped.
• Block the tracks found on both sides of the door is you want to leave the door open.
• Unplug the door opener so the door won’t budge.
If you install the spring incorrectly, it may cause severe problems. Take note that the door springs are attached to the door itself so if it fails, the door will crush everything underneath it. You should contact an experienced garage door repair professional to work on your problem,