If the operator is equipped with safety sensors (electric eyes) they must be in alignment and connected, when they are not, the result is typically what you are describing. What brand of door opener do you have?
It's a Chamberlain Professional 1/3 hp. The eye is lit green and when I block it, it goes out, then come right back on. I'll double check that since the kids could have easily "bumped" into it.
