I just got through reading the book "one second after" by William R. Forstchen. Its about the events following an EMP event - power failure, food shortage, looting, breakdown of civil order. Very interesting reading.
If you are unfamiliar with what an EMP is and what it can do, basically, it fries computer chips causing failure of the component whether its controlling the computer and ignition in your truck, a nuclear power plant's controller systems, or radios, tvs,...etc. They are produced by nuclear detonations. Wikipedia has a very good article on the subject.
The sun can also produce this same effect as an emp that can fry solid state electronic components as well. These events are called coronal mass ejections. They are rated on an intensity scale by nasa. The earth's magnetic field can shield from most of them, but it is possible that an extraordinarily intense one can overcome this and cause major problems. We just had a big one a few weeks ago that was right in the center of the scale, which is a rather large one compared to normal sun activity. It was such a cause for concern that they wrote an news blurb about it; but as it turned out, the earth took care of it fine.
IMHO this is probably the most realistic event that could actually cause the sort of SHTF scenario we see in the movies. It would place us back into the 19th century overnight; as well as cause the demise of millions and millions of humans in a 1st world country like ours. The scary thing is that one of these massive ejections could have happened at any time before the advent of solid state electronic components (maybe around 1970) and we'd have never known it happened because it wouldn't have had any observable effect on anything biological. Today we have the SOHO, which is a set of two satellites that monitor the sun's activity 24/7 and can detect and measure the intensity of these CME's. When one occurs, we have about 4 days notice before the wave gets to earth. Just enough time to go to town and get some provisions and gas, and kiss your air conditioning goodbye -- IF they announce it to the public, that is.
The only protection I've read about that can protect your personal electronic equipment is to create a faraday cage around the items. A metal trash can is what I use for my radios and gps unit. Just FYI.