Roadracer, by far the most difficult (and longest) to recover from would be a massive EMP from a nuclear weapon designed for that purpose (or from a massive solar storm with a coronal mass ejection – CME – that hit the earth squarely). The devastating and long term problem would come from the fact that power transformers would literally have their cores melt (the windings inside the transformers). The very large ones that you see at power sub-stations (not the big “cans” on the tops of neighborhood power poles) are not stockpiled. And if my understanding is correct, none were even manufactured in the U.S. until just the past few years (if they’re even in production yet). Power generation would be impossible, therefore manufacturing OF the parts necessary to start generating power again would be impossible – a vicious circle. We need power to operate the production facilities, but we don’t have power because we don’t have the pieces that the production facilities would produce if we do. We’d instantly be in the early 1800s again, and no way to recover for many years.
For the most part, any electronic equipment with solid-state circuitry in it is vulnerable, so even if many of the solar panels survived (there is some evidence, supposedly, that they might), there would still be no controllers or inverters, so we’d be stuck with low voltage DC. So a massive CME or HEMP is the real nightmare scenario (assuming we didn’t get hit by a large number of highly radioactive nukes that spread fallout nationwide – but then that would go world wide as well).