With temperatures this cold, your windshield wiper fluid lines may become frozen. Some fluid may still be in the reservoir that is not sufficient for below freezing temperatures. When the lines freeze, the system may incur damage, so you must resolve the issue promptly.
Here’s how:

1. Fill a pitcher with hot (but not too hot) water. If it is painful to touch, it is too hot, and it could crack the reservoir on the tank.
2. Lift the hood of your car, and ensure it is stable by propping it up with the available bar. Remove the cap to the reservoir.
3. Pour one cup of the hot water slowly into the reservoir and let it sit for several minutes.
4. Using a turkey baster, suck out fluid and discard it. Do this until there is no more fluid to remove.
5. Pour another cup of water into the reservoir and remove fluids again with the baster. Continue this process until the tank is completely empty.
6. Fill the reservoir with wiper fluid that is specifically made to withstand freezing temperatures. Purchase a variety that can handle -20 to -25 degrees Fahrenheit; this should be sufficient.
7. Locate the hoses than run the fluid from the reservoir to the expenders that spray the fluid onto the windshield. Gently blow air from an electric hair dryer over the hoses. Do not blow the air on one spot for too long, as this could melt the hose. Do this for about ten minutes.
8. Start your vehicle and press the switch to spray washer fluid to make sure it works.

Stay warm, folks!
 
Courtesy of eHow
http://www.ehow.com/how_5997048_thaw-windshield-washer-fluid.html