So this is going to depend quite a bit on just what they started with for a planet, what the terraforming process was, and how much of their knowledge was lost (as opposed to just infrastructure destruction.)
It's possible that they have a fair-sized cache of immature fossil fuels that would still work reasonably well.
Look at it this way. Fossil fuels are the remains of ancient plants (and animals, but mostly plants) that died in such a way that no bacteria or fungus ate them. Then they sat around for a while and all the water and such evaporated leaving big, long chains of carbon. (A bit oversimplified, but sufficient for our purposes.)
To terraform a world like, say, Mars where all the oxygen is locked up in various oxides (and especially CO2) You're going to need to free that oxygen somehow. There are lots of ways you can do this, but when you release the oxygen into the atmosphere for people to breathe, you're going to be left with a big pile of whatever the oxygen was bound to just sitting there.
Pull your oxygen out of the iron oxide, you'll have a whole bunch of cheap iron. Pull it out of SiO2 and you'll have lots of material to make computer chips from.
Pull it out of CO2 and you'll have a massive pile of carbon left sitting there. Big chunks of pure (or nearly so) carbon are commonly called "coal".
So the question is what did they get the oxygen out of for what byproduct they have sitting around, and how did they do it for how accessible is that byproduct.
If the world had water, but no life, and they just seeded algae into the oceans to free up some oxygen (even here on Earth most of our oxygen generation comes from the oceans) then they'll have a whole pile of dead algae corpses on the bottom of the ocean slowly turning into coal and oil. The natural refining process takes quite a long time, but speeding it up really only takes a pressure cooker. (This is how "synthetic" oil products and charcoal are made.)
If there's not much for water and they used land-based plants then they don't even have to scrape the muck off the ocean floor, they're just making charcoal and/or biodiesel. Hydrocarbons are about the most efficient energy storage we have by weight and by volume, so it will definitely still make sense to use them, especially at the lower technology levels.
Even once they have photovoltaics or solar-thermal generators the fact that plants are, by comparison, super cheap per square foot to sow and harvest (and they stash away energy in nice, convenient, portable, long-term hydrocarbon storage) is going to mean they continue to be an attractive option. Especially where, by then, they probably will have developed specialized varieties to maximize output and purity of the rendered oil and charcoal.
So basically the answer is that, without a large reserve storage of energy that was banked up millions of years ago, they're going to be devoting a lot more of the surface of the planet to agriculture, specifically to grow things that produce a lot of combustibles in a small volume. Precisely what this will be will depend on the actual climate of the planet. Peanuts, sunflowers, various kinds of tree, possibly massive algae farms -- the efficiency of chlorophyll is about the same regardless, so it's just a matter of finding a species that thrives and stores its energy in an easily-extractable manner.
Anything else probably takes some kind of "magic" since we don't really know of a non-hydrocarbon based path between the stone age and modern technology.