Check Engine Light - 2004 Toyota Prius?

I have a prius that I took to the Toyota dealer just a month ago and they said that the car was in perfect condition with no machnical problems to report. Then a few days ago a large snow storm hit and the car was sitting outside in really cold weather. The next morning I went to turn it on and the Check Engine light came on. I've had the car for a year and it's never come on before until now. The vechile seems to be running just fine with no funny noises or smells to indicate any problems. Does anyone have any ideas on what it might be?? I don't know if the really cold weather from the snow threw somthing off and thats why. I would love to take it back to the dealer, but really don't want to pay $100.00 an hour just for them to tell me it's something simple that I could fix myself. THANKS!