Why would we regress to pre-industrial level of technology? Even if you somehow wipe 90% of the population worldwide, this brings the population back to mid-18th century levels - which is to say, when industrial revolution was already ongoing. But that same number of people would know all the things that had to be discovered back then, and would still have a lot of machinery, and a lot of already-refined materials, to bootstrap from.
Nor is there a particular shortage of hydrocarbons to burn, with consumption reduced so much due to population loss and reduction in quality of life. Consider that US today emits more than 200x carbon into the atmosphere than it did in 1850, and that this growth has been exponential. So what we consider one year worth of reserves today, could provide energy for many decades in this hypothetical.
Nor is there a particular shortage of hydrocarbons to burn, with consumption reduced so much due to population loss and reduction in quality of life. Consider that US today emits more than 200x carbon into the atmosphere than it did in 1850, and that this growth has been exponential. So what we consider one year worth of reserves today, could provide energy for many decades in this hypothetical.