It would probably sound like a harmless stroke of distant lightning and suddenly all lights would go out. Cars, trams and railways would be at a standstill, every radio, every MP3-player and every mobile phone would fail at the same time, burned cables, overloaded batteries, and your PC with all your saved data would be a useless heap of plastic, hot wires and metal. To cut a long description short: Within the target area the whole electronics-based civilisation of a big city would be thrown back to the level of the Stone-Age. "Rien ne va plus!"
The technical abbreviation behind this nightmare-scenario of the modern information age is "EMP" (Electro-Magnetic Pulse). In 1932 a Bulgarian scientist found, that explosive charges not only emmit light and sound, but also electromagnetic pulses, just like thunderstorm flashes do. These pulses cause inductions in wires (like a magnet of a generator), which can generate short impulses at very high voltages. But, like the range of a flashlight the sphere of diameter is relatively short.
It is known that the USA and Russia have been fiddled around with such "E-Bombs" since the 1960's. Probably many other nations do it secretly, because the weapon (harmless for human beings) is as economical to produce as effective in paralysing the enemy. The USA demonstrated it during the "Kosovo-War" and they probably used it in the Iraq, too. Carlo Kopp, an Australian expert in high-tech warfare thinks that producing a simple "E-Bomb" would not cost more than 2000 Euros. For the construction itself the electronics-standard from the 1940's would be enough and besides Semtex, the needed plastic explosive, all components would be freely available on the market. The only thing that gives the free world an additional breather is, from the terrorist's point of view, the knowledge of an effective construction.
A fully developed "E-Bomb" – it is assumed that in the meantime, the USA is working with special superconductors – exceeds the electromagnetic field strength of a thunderstorm thousands of times. It is able to induce up to one million Volts into every wire, cable and even into stretches of train track. As a general physics rule you can assume, that the longer the wire, the higher the induced voltage will be.
The only protection against these electromagnetic fields offer so-called "Faraday cages", a construction made of wires. Inside this cage, a computer, for example, would be relatively safe as long as there is no metallic connection to the outside world.
So the modern civilisation would be helpless in the face of a calculated "E-Bomb" attack. The only thing you can do is to periodically save your data on CD or DVD (which you should be doing anyway), to cope with the bomb's effect of destroying your data saved on your PC. Although CDs have little metal content, they are assumed to be immune to electromagnetic terrorism.