It is difficult to say, as there are alternatives to oil available. Would it drastically alter the way in which we generate energy and live our lives? I would say, of course.
However a move away from fossil fuels doesn't have to mean a decline in living conditions long term, it means a fundamental change in how we both perceive what is important and what we need, at least until other energy sources can be retooled and implemented.
I would expect that after a century or two of interregnum we would actually come out of it better off, especially as it would mena the end of the Oil economic structure that Globally brings so much strife to the world.
Going back to the 'Dark Ages', if you want to see what kind of accomplishments happened during this period, next time you are near one of the Great Cathedrals, (I see you're a Swindonian, so Salisbury is a good example) go inside a look, and I mean really look, at how it was built and what knowledge and artisan-ship was needed to build a structure that has lasted for almost a thousand years. No backward society could have constructed such architectural marvels, especially as they invented the tools, mathematics and architecture as they went along. The Magna Carta is also in Salisbury Cathedral and that is the basis of our legal system, again from the 'Dark Ages'