After returning from dinner out one evening, my wife and I entered the house and then heard a loud noise in the garage. We re-opened the door and looked in and saw nothing out of place; no shelves crashed down nor garden tools fell over.
The next morning we found out what happened. The automatic garage door opener would not open the door more than about a foot and then it reversed. I disengaged the opener and tried to open the garage door manually, but it seemed to be stuck. Then Deborah noticed that the right torsion spring above the door was broken in two.
I described how I replaced the springs (with video) at this web page: http://www.webguild.com/DIYClinic/.