Solar Flare
While it is true that there is no way to know exactly what will happen if a solar flare, or more precisely a CME (coronal mass ejections), hits the earth, but we do have history to give us an idea of what could happen. Below are two recent quotes and a video about CME's and their potential and proven effects.
"People of the 21st century rely on high-tech systems for the basics of daily life. But smart power grids, GPS navigation, air travel, financial services and emergency radio communications can all be knocked out by intense solar activity." "A major solar storm could cause twenty times more economic damage than Hurricane Katrina, warned the National Academy of Sciences in a 2008 report, "Severe Space Weather Events-Societal and Economic Impacts."
While I personally believe that they (government and astronomical professionals) are being responsible by not informing the public of just how severe these could effect our way of living, I also believe that it is prudent that those who take this into consideration and truly contemplate the extent of disruption to life that this type of event could cause, and be prepared. The first and most important issue to be concerned with is the potential for this type of storm to blow up transformers, which the majority of major electric companies own is about 20-30, for which they only have one or two spares available at a time. If the majority of the transformers blow up then the public would be without power, which besides being annoying can be the start to disaster. Imagine what in your life would be affected by a loss of electricity: air conditioner (heat related issues), refridgerator (food spoilage), the pumps that supply your water (limited source of drinking water), gas pumps (no gasoline), traffic lights (accidents), ATM's (no cash), radios and televisions (no warnings), telecommunications (no way to contact family members or help), hospitals without electricity thus operating on back-up generators which will eventually run out of gasoline to operate, security systems shut down (looting), and evenually social disorder.
In the past, 1889, a CME hit the earth and shut down telegraphs and caught telegraph buildings on fire. Back then people did not depend on such things as electricity, and they were more used to living the simple life off the land. People today are not only dependent upon electricity, but on the ease of getting their food and comfort from stores. Those who decide to learn to limit their dependency on these comforts will be the ones who have the best chance to handle the loss of these comforts. As I said before, no one knows exactly what will happen but being oblivious to the potential is worse than ignorance and borders on stupidity.