Before industrialization and discovery of oil/gas/electricity, what were the equivalent of energy and critical resources? Did it remain the same or change depending on era? (antiquity, medieval, etc.)
Did grain qualify as such a resource for the Romans?
A lot of the answer would depend on how you mean "energy-critical resources." Looking at total energy use- we use more total food energy than we ever did before, and the same is largely true for biomass (though we use it differently).
https://ourworldindata.org/energy-mix
For activities like forging and smelting, charcoal was a common high-density fuel used in medieval and early modern Europe. The process for creating charcoal was both labor-intensive and material-intensive, but the result burned far hotter and far cleaner than the wood from which it was created; cleaner, in fact, than much of the coal that later replaced it. (The flip side, of course, was that it was easier to get coal in quantity).
Videos of the early modern process can be found online.
Wood itself was usually carefully grown and not collected wild; the trees were placed in managed forests, then carefully coppiced or pollarded. The need for firewood also competed with other uses, such as building materials. There were strict limits on how much people could take; a common complaint in medieval chronicles was in the stinginess of landowners. Despite this, the rising need for wood led to increasing deforestation, which helped trigger the switch to coal in England at the beginning of the industrial revolution.
A Medieval Book of Seasons, Marie Collins & Virginia Davis