Having an automatic garage door is advantageous for some reasons. You do not have to bother to manually lift up and pull down the heavy garage door, imagine how troublesome it would be without a device that could help you to do the task every day. The garage door will automatically open and close in an instant clicking a remote button. However, when you experience a power outage, you will no longer make use the advantage of the device. During the power outage, you can still manually open the door with the release mechanism of the device.
You need to find the release mechanism; usually it is located on the track route between the motor and the door. You might find drive carriage and a release mechanism along with a cord hanging. Pull the handle to make the carriage free and until it stops; a little force might helps especially pulling down an older opener. Do not continue to open the door manually if you cannot let free the carriage otherwise you will do harm to your opener.
After the carriage is released, you can manually open your automatic garage door. The door should go up to fully open or go down to close freely without significant problem. In the situation that your door can be manually open or close, you might want to find a temporary solution such as using a latch to let no one able open the door from outside the garage.
You need to re-engage your automatic garage door opener back to its place again once the power is restored. Back to the track route between the motor and the door to pull back the release mechanism cord until it is completely open. After that, try to raise or lower the door manually until the carriage locks to its place. Operate the garage door opener to verify the opener is working as supposed to be.