A society which can terraform worlds isn't suddenly going to go back to the bronze age - many of the artifacts which were used to terraform the world and maintain it's climate, atmosphere and hydrosphere should still be available, and more importantly, running, otherwise you might suddenly discover your planetary ecology is suffering system and cascade failures as it reverts to the more stable equilibrium of pre terraforming times.
Orbiting mirrors. These would be redirecting sumlight to or away from the planet
But if we hand wave this - perhaps reverting to a more stable equilibrium will require thousands of years, then there are still several potential work arounds.
Abiogenic hydrocarbon formation. While according to this theory you still need an active core and mantle for hydrocarbons to form and accumulate, making Mars a poor candidate, if you are willing to use this idea in your story, there will be deposits of hydrocarbons that can be accessed by drilling equipment.
Agricultural fuel sources. Plants can be harvested for oils, which can be used directly as fuel and chemical feed stock, or the oils can be converted via chemical processes into biodiesel and similar fuels. Alcohols can also be fermented from plant materials, and of course dry biomass can also be used directly as fuel - look up wood gasification and it's use in powering cars during WWII.
Genetic engineering. Some plants already produce oils which can be harvested for industrial use. Genetic engineers can modify these plants for higher outputs (although generally at the cost of some other attribute), or plants can be cross bred in the traditional way to produce more oils.
Of course other sources of energy exist as well, like wind or geothermal, but as the writer, you can decide which idea or combination of ideas would work best.
Spanish windmills from the middle ages