Rav Shemuel Pinhasi (contemporary scholar in Israel), in his work Ve'daber Davar (13:5; listen to audio recording for precise citation), rules that one may not partake of food that a gentile cooked for a Jew on Shabbat. Since the non-Jew cooked the food specifically for a Jew, nobody - neither the person for whom the food was cooked nor others may eat the food until after Shabbat. Rav Pinhasi adds that this applies even if the food was edible raw. He further notes that the food is deemed Mukse, and thus one may not move it on Shabbat. (This ruling is cited as well in Shemirat Shabbat Ke'hilchata 30:39 in the name of Rav Shlomo Zalman Auerbach.)
Rav Pinhasi writes that in a case where a Jew instructed a gentile to cook food for him on Shabbat, in violation of Halacha, he can correct the wrongdoing by feeding the food to an ill patient. Halacha allows asking a gentile to cook food on Shabbat for the purpose of a "Hole She'en Bo Sakana" a person who has taken ill, even though the illness poses no threat to his life. Hence, if one did ask a gentile to prepare food on Shabbat, he should endeavor to correct his wrongdoing by finding an ill patient to whom he can feed this food. (Hacham Ovadia Yosef likewise mentions this Halacha in his work Yabia Omer, vol. 2, Siman 26.)
If one's electricity short-circuited, such that his hot plate or other food-warming devices lost power and the food became cold, he may not ask a gentile to restore the electricity. This applies even if the food had been completely cooked. Rav Pinhasi writes that one may not even ask a gentile to ask another gentile to restore the electricity. (Hacham Ovadia Yosef issues this ruling in his work Halichot Olam, vol. 4, p. 137.)
If a gentile acted independently and restored the electricity, then one may not partake of the food until it completely cools, as he may not benefit from this act of Melacha (activity forbidden on Shabbat) performed by a gentile for a Jew. And if a Jew asked the gentile to restore the electricity, one may not partake of the food until after Shabbat.
Summary: One may not ask a gentile to cook for him on Shabbat, or to restore the electricity if the power to his hot-plate or other food-warming apparatus short-circuited. If a gentile cooked for a Jew on Shabbat, the food is forbidden for consumption until after Shabbat. If a gentile restored power to a food-warming appliance after the food had cooled, one may not eat the food until it cools again. If a Jew had asked the gentile to restore the power, the food may not be eaten until after Shabbat.
