Garage door blues

No, not the latest country hit... my automatic garage door has decided it doesn't want to close. It opened OK when I came home with some groceries. And then it refused to close. It would move about a foot or so, then stop and reverse. I turned it off and closed it manually, but now I have to figure out what's wrong. Luckily my car is outside, so I won't have to futz with the door if I want to go out.

*&^%$#@ machinery.

11:20 pm

OK, so I'm an idiot. An empty mulch bag fell off a pile of full bags into the path of the sensor that triggers the door to reverse to keep from squishing errant pets or kids. Duh.

Stupid garage door can't tell a plastic bag from a dog...