Openers today are required to have an emergency release cord to quickly override the automatic function in the event of an emergency. This release cord is also helpful in the event of a power outage - you'll still be able to get your door open and shut manually until the power comes on again.
Open your overhead door and press the button to bring it down again. Place something in the path of the door like a broom and check to see if it reverses. If it doesn't, this could indicate a problem with the sensors or the sensitivity of the safety reversal feature.