We'll use a nearby lightning strike as an example, although it is just one of countless problems that can strike your system. Lightning strikes a nearby transformer. If the surge is powerful enough, it travels instantaneously through wiring, network, serial and phone lines and more, with the electrical equivalent force of a tidal wave. The surge travels into your computer via the outlet or phone lines. The first casualty is usually a modem or motherboard.
Chips go next, and data is lost.
The utility responds to overvoltages by disconnecting the grid. This creates brownouts and blackouts. If the voltage drops low enough, or blacks out, the hard disk may crash, destroying the data stored on the disk. In the worst case, password protection on the hard drive can be jumbled, or the file allocation table may be upset, rendering the hard disk useless.