They will be fine for at least several hours. Some people here have had power out for a day or more and their tanks were fine.
The two most important things are temperature and flow. Try to keep the tank's temperature normal. If you have to, wrap it in blankets or comforters to slow the loss of heat. Keep your house as warm as possible (i.e. don't keep going in and out the door).
With flow, you want to keep the water moving. Even if you have to manually stir it with a wooden spoon or something every 10 or 15 minutes.
Without flow, the tank will still be fine for several hours. Don't worry about lighting -- it can go for days without lighting.
Many people with reef tanks invest in a small generator in case of a power outage. Just to run the heater and powerheads. You can also get battery operated powerheads, an inverter that hooks to your car battery to power things, or a computer battery back up system to run a few small items.
How long has yours been out?