If there is a power outage, the freezer fails, or if the freezer door has been left ajar by mistake, the food may still be safe to use. As long as a freezer with its door ajar is continuing to cool, the foods should stay safe overnight. If a repairman is on the way or it appears the power will be on soon, just don't open the freezer door.
A freezer full of food will usually keep about 2 days if the door is kept shut; a half-full freezer will last about a day. The freezing compartment in a refrigerator may not keep foods frozen as long. If the freezer is not full, quickly group packages together so they will retain the cold more effectively. Separate meat and poultry items from other foods so if they begin to thaw, their juices won't drip onto other foods.
To determine the safety of foods when the power goes on, check their condition and temperature. If food is partly frozen, still has ice crystals, or is as cold as if it were in a refrigerator (40 °F), it is safe to refreeze or use. It's not necessary to cook raw foods before refreezing. Discard foods that have been warmer than 40 °F for more than 2 hours. Discard any foods that have been contaminated by raw meat juices. Dispose of soft or melted ice cream for quality's sake.

Additional Resources:

http://www.fsis.usda.gov/Fact_Sheets/Focus_On_Freezing/index.asp#16