Sun giveth, sun taketh away

The world is already unstable. The economy is tanking, unemployment rises, public discontent as well. A badly timed solar flare knocks out a large proportion of telecommunication satellites and terrestrial power distribution networks; cascades of failures (redundancy? What redundancy?) blackout entire countries for days, some areas for weeks. Cities, now full of people and with disabled infrastructure, in few days break into chaos. Global economy, losing its main financial centers and with most of its communication lines severed, undergoes cascade failure; some of the centers recover but too late to resume business as usual. Latent tensions between several nations escalate into regional wars, as leaders fueled with ambitions and wishful interpretation of incomplete intelligence seize perceived opportunities, and cascade out of control like a bar brawl; limited but sufficiently intense nuclear exchanges (Pakistan vs India, perhaps Israel joining the fray and nuking somebody too? Can China get involved? ) may take place and cause a runaway in already destabilized weather patterns. Biological and chemical weapons may be used regionally as poor man's nukes. Nuclear winter sets in, with devastating results for agriculture, resulting in widespread famines and further intensifying conflicts, adding food and water into motivations for fighting.

Couple years later, the world is a patchwork of relatively working city-state regions, Somalia-like failed states, glass parking lots, and deserted shells of cities. Technological development continues in enclaves with resources, with much larger allowances for risk and side effects as the stakes are higher now. Genetic/technological modifications of people aren't uncommon, mostly for military/paramilitary purposes. Areas low on technology are rich on violence, areas rich on technology are under constant surveillance as the power structures are attempting to reestablish themselves and prevent the always threatening order collapse.

Good, bad, that depends on what side you're on. Each is just attempting to survive instead of the other one.

There should be a few hinge points in the history on which the breakdown depends, which can be interrupted - whether by talking a person out of some action, blowing up some system in advance in order to prevent its use to escalate a conflict or to prevent it yielding data used to start one, etc.; a few power lines may be severed, e.g. by blowing up a couple high voltage pylons, to force a power station offline during a critical time to prevent more serious solar flare related damage to its systems, speeding up region's recovery to avoid cascading into a failed state. A not-exactly-important person may have to be killed before gaining importance. An information may be given at right time to right person to alter their decision. A phone switchboard may have to be blown up or forced to fail other way so a crucial communication does not happen. A reactor may be forced offline so the transformers are not energized when the flare comes. A redundant power line has to be severed so during the following cascade failure a region gets disconnected from the grid (so called islandization) instead of being pulled down like the others. Details like these, relatively small changes in a number of predetermined points in space and time.

The hero may not have any really special superpowers; perhaps just eidetic memory, talent for handling high-complexity systems, and ability to see their hinge points where small changes have disproportionally larger effects.