pcns

Make sure the PC is plugged in.  Though this may sound obvious, if you have small pets that like to curl up behind the PC and stay warm, it happens.

Remove the power cord from the PC and plug it into another device, such as a power brick for a laptop, printer, or another peripheral. Not all cords are interchangable, keep this in mind.

Make sure you haven't unplugged any surge strips.  If you have a battery backup or power strip, make sure it's functioning and providing power to the computer.   In some businesses we've seen customer plug space heaters, microwave ovens, laser printers, and other high current devices into the power strips or battery backups, and it trips the internal circuit breaker.  With some battery backups, when their lead acid batteries deplete and no longer hold a charge, they will stop supplying power to the A/C receptacles.

The most likely cause of a dead PC is a failed power supply.  If you smell smoke, heard a loud pop, or hear a faint ticking sound from the rear of the PC you should unplug your PC immediately and seek professional assistance!