I suppose the obvious answer would be, pray! But I have short power failures and never have a problem. If more than 6 hours were realistic, as in a storm, then I would consider having 2" styrofoam pieces to close in my aquarium, direct on the glass, and at least one on top! Heat rises.
Other than this, I can think of nothing short of a portable gas heater to maintain room temperature.
If the temp in the aquarium falls, raise it again slowly. Don't try adding heated water, this tends to kill things off.
Jamie