"We're not sure if we could get back to our current tech level afterwards" isn't an xrisk.
It's also purely speculative. The world still has huge deposits of coal, oil, natural gas, oil sands and shale oil, plus large reserves of half a dozen more obscure forms of fossil fuel that have never been commercially developed because they aren't cost-competitive. Plus there's wind, geothermal, hydroelectric, solar and nuclear. We're a long, long way away from the "all non-renewables are exhausted" scenario.
"We're not sure if we could get back to our current tech level afterwards" isn't an xrisk.
Yes it is. Right now, we can't deal with a variety of basic x-risk that require large technologies. Big asteroids hit every hundred million years or so and many other disasters can easily wipe out a technological non-advanced species. If our tech level is reduced to even late 19th century and is static then civilization is simply dead and doesn't know it until something comes along to finish it off.
...The world still has huge deposits of coal, oil, natura
The FHI's mini advent calendar: counting down through the big five existential risks. The first one is an old favourite, forgotten but not gone: nuclear war.
Nuclear War
Current understanding: medium-high
Most worrying aspect: the missiles and bombs are already out there
It was a great fear during the fifties and sixties; but the weapons that could destroy our species lie dormant, not destroyed.
But nuclear weapons still remain the easiest method for our species to destroy itself. Recent modelling have confirmed the old idea of nuclear winter: soot rising from burning human cities destroyed by nuclear weapons could envelop the world in a dark cloud, disrupting agriculture and the food supplies, and causing mass starvation and death far beyond the areas directly hit. And a creeping proliferation has spread these weapons to smaller states in unstable areas of the world, increasing the probability that nuclear weapons could get used, leading to potential escalation. The risks are not new, and several times (the Cuban missile crisis, the Petrov incident) our species has been saved from annihilation by the slimmest of margins. And yet the risk seems to have slipped off the radar for many governments: emergency food and fuel reserves are diminishing, and we have few “refuges” designed to ensure that the human species could endure a major nuclear conflict.