One of the many side effects of detonating nuclear weapons is something called an electromagnetic pulse. Basically, it's a burst of energy that knocks out most electronics in the vicinity.
Over the decades since EMP was discovered, we've managed to make gadgets that do the same thing without the nuclear devastation part of it. You might remember the "pinch" device from Oceans 11 that the crew used to temporarily knock out the power to Las Vegas. That scene wasn't too far-fetched, but a real EMP blast could be much worse because the effects aren't temporary.
A properly made EMP bomb actually fries unprotected computer processors, and typically only select military and government computers are shielded against it. In our modern era where computers control everything, an EMP blast could knock down our power grid, communications, financial systems, manufacturing, payment systems, the Internet, hospital equipment, computers, smartphones, tablets and basically bring the entire country to a standstill until new systems could be brought online.
So it's no surprise that countries are working on first-strike EMP weapons as a way to take an opponent out of the fight before the fight starts. Boeing has an EMP cruise missile called CHAMP (Counter-electronics High-powered Microwave Advanced Missile Project) that it's worked on for the last few years.
In tests, it actually knocked out a military compound just by flying past. It's likely that other major powers, such as China and Russia, are working on similar systems, or full bombs that could affect an entire country or continent.
The reason we bring this up is that former security software creator, self-proclaimed presidential candidate and sometime-fugitive John McAfee recently had some thoughts on the subject in post at International Business Times. He claims that EMPs and cyberwarfare are going to completely knock out America's electronics. Even worse, he claims that after two years without power, 90% of the population, or 270 million people, would be dead.
While losing power and electronics would certainly be a disruption to modern life, and some people would die from it, that figure might be a little excessive. There's also the assumption that power would be permanently gone. Replacing the command and control systems for the country's infrastructure would be a long and expensive process, especially if the replacement systems were shielded and more secure than current systems, but not impossible.
While the possibility of this entire scenario is remote, it doesn't hurt to make preparations. Even if a national emergency doesn't happen, local disruptions from cyberattacks are still a possibility. It's a good idea to be prepared just in case.
If you're interested in how future wars might be fought with technology, check out Kim's interview with an expert on the subject.