What If It All Goes Smash? --The Short Answer

You're going to die.

Joe addresses it in more detail but the 30-second read is, if technological civilization went belly-up tomorrow, unless you're Amish or a very serious survivalist, you're going to die (and they're going to be on short rations and will face lifestyle changes; note all the "low-tech" factory-made goods!). If you're a Prepper or a good LDS member (etc.) and relatively lucky, it might not be for a long time -- a year, maybe two. Most of us less-ready types will be toast within six months and the remainder will go like mayflies in the first Winter.

Thoreau in his cabin was dependent on technological civilization -- his axe, his clothing, the means to make fire. Even assuming you are as clever and resourceful (not to mention fictional) as the boy in My Side Of The Mountain, he wasn't competing with several thousand other hungry survivors. (Short-term advantage goes to the meanest.)

Most of the human race lives in climates that will kill us if we lack adequate protection from them and we do so at population densities that are insanely unsustainable for hunter-gatherers. And the really low-level skills that will keep you housed and clothed, fed a sufficiently-balanced diet to prevent malnutrition are not widespread. Depending on how things go -- how bad the die-off is -- there's a fair chance the second year will start with a better availability of salvaged goods; but I wouldn't count on it, as there are a lot of ways for a few fools to destroy a city, starting by being foolish with fire.

--Of course, that's not especially realistic, barring EMP from natural or man-made sources or something similarly unforseen; most of the really big TEOTWAWKI events result in widespread death along with the destruction, which takes us back to "See first paragraph" land.

So what's realistic to plan for?

-Economic Collapse. Something like what happened in Russia, only without the localized booms. Or we're all in Cuba, only the "classic car" you're gonna have to keep running forever is a Prius. Good luck! This calls for Depression-era coping skills and starting with a very full larder will help (in fact, it's never not a help, unless persons inclined to take it from you find out about it). Personally, I rate this the most likely and figure I'm likely to spend my retirement in a third-world country without having to move to one. Slow collapse is probably better than sudden and, horribly enough, may actually be what the market-regulators are really aiming for. (I'm lookin' at you, Ben Bernanke).

-Terrorist Activity. From "dirty" bombs to chemical or biological agents or just exploding bus stations, this would be horrible but also tends -- mostly! -- to draw people together. Coping skills would be more like the Home Front skill set for the U.S. or Canada in WW II. OTOH, regional/political violence would be differently polarizing and, Tom Leherer being too good a prophet, what do you do if the Crips or Bloods have The Bomb -- or weaponized smallpox? I guess we might as well lump "civil unrest" at the low end of this category; a good-sized riot can have very much the same effect, only on a smaller scale; and that's the most probable one you'll encounter.

-Natural Disaster. New Madrid Earthquake, anyone? Aw, why stop there -- Yellowstone? Okay, those get close to the "high probability of death" level but they're outliers; smaller quakes and eruptions, flood, hurricane, tornado, big fires and so on are a lot more common.

Dean Ing talks about the need to consider all the higher-probability events and map out what you'd do -- including literal maps, marked for the likelier scenarios. If things get bad enough you need them, don't count on your ISP or smartphone to be able to dig 'em out -- and you may want to be discreet about that GPS.

The big, flashy possibilities are the "sexiest" to plan for (sheesh, lookit even my "smaller" examples) but they're outliers; the littler ones are more likely to be the ones you'll face. And by a happy coincidence, they're also the ones with the best return on investment, too. If it goes Mad Max, you may be better off with a colander than canned goods; but if it goes LA riots or blizzard of the century and you need to hole up for a week or three, that stack of MREs will come in handy.