I'm writing a story in which a world has been terraformed into an Earth-like planet and settled by a group of colonists. However, a cataclysm forces society to regress to the Stone/Bronze Age, at which point they basically have to restart their development from 0.

My question is, without prior eons of life forming fossil fuel deposits, would there just be no fuel to power industrialization or other societal advancement? What alternatives might they use which could be comparable?