The question occasionally arises in situations where a power outage or disruption of gas supply turns off one's oven or stove on Shabbat, whether it is permissible to ask a non-Jew to relight the pilot or turn the appliance back on. This question becomes particularly pressing in catered events, such as in hotels or Shabbat morning affairs in synagogue halls, where many guests are in attendance and will have only cold food to eat if the oven or stove cannot be rekindled. The issue can also arise in one's home, when company is expected and the host will have only cold food to serve them. One might have argued that in the interest of Kevod Shabbat the honor of Shabbat it should be permitted to ask a gentile to turn on the appliance.
However, Chacham Ovadia Yosef, in his work Halichot Olam (vol. 4, p. 137), writes explicitly that it is forbidden on Shabbat to ask a gentile to turn on one's appliance. Turning on an appliance constitutes a Torah prohibition on Shabbat, and Halacha therefore forbids instructing a gentile to perform this act, even under the circumstances described above. It is likewise forbidden to ask the gentile to ask another gentile to turn on the appliance. Furthermore, Chacham Ovadia rules that if the gentile does turn the appliance on to heat the food, one may not derive benefit from his action. Therefore, one may not partake of hot food, and must instead wait for the food to cool, so that he does not derive direct benefit from the action performed by the non-Jew. And the individual who had asked the gentile to turn on the appliance may not partake of the food at all, even after it has cooled.
Summary: One may not ask a gentile to turn on one's oven or stove to heat food on Shabbat, even if he will otherwise have no hot food for his Shabbat meal. If a non-Jew does turn on the appliance to heat the food, one may not partake of the food until after it once again cools, and one who asks a gentile to turn on the appliance may not partake of the food at all, even after it has cooled.