If the power comes back on the opener might attempt to open the door while it is disengaged which can make it harder to reconnect.
Pry garage door open.
If you do not have an entrance door to your garage either to the interior of your house or the exterior there should be a key based garage door release on the outside of your garage.
Ever forget your keys to the house or even lost your garage door remote.
Garage door locked from the outside.
When you open a garage door by hand make sure to stay safe.
On either side of the garage door place two 6 foot ladders.
Get two crowbars or pry.
In order to do it properly and safely you can follow these steps to open your garage door with a broken spring.
Unplug the automatic door opener.
These openers use a threaded rod to push and pull a trolley that lifts and lowers your garage door with ease.
You would obviously need the key for this but it would release the garage door without resorting to prying it with a hanger or another similar hack.
Unplugging the garage door opener before you try to manually open or reconnect your garage door will keep you safe.
These ladders will help keep the garage door open when you manage to prop the door open all the way.