We all have those moments when something just won't work. It might be something major, but chances are that it's really something simple. Rule number 1 — don't panic. Step back, take a deep breath and start looking at things logically.
Is it a peripheral like a printer that's not responding? Check the connecting cable — both ends. They can work loose. Just because a cable looks like it's connected doesn't mean that it is seated completely. Check the power plug. If it's plugged into a power strip, is the strip turned on? Has the strip developed a bad socket? Ditto if you are connecting through a USB or Firewire hub. They can also develop bad ports. One more thing — is the power strip/hub/etc. connected to a wall outlet that is tied into a light switch? And yes, I've seen every one of those things happen over the years.
If a printer is active and the computer sees it, check the ink and paper supplies. Yes, it should tell you when it needs supplies, but trust me, they don't always do it. Some inkjet printers will refuse to print, even black text, when it is out of one of the colors.
If the computer will not turn on, check the power cords, just like you would for a peripheral.Try plugging it into a different receptacle , preferably on a different circuit. In my home office, each wall is on a separate circuit. Try resetting the System Management Controller (SMC). On an Intel iMac you do it by unplugging the power cord, press and hold the power button for 5 seconds, then reattach the power cord. If that doesn't work, a service call is probably in order. If the computer turns on but turns off after a while, it could be a matter of overheating. Check to be sure the vents are not blocked. Blocked or dust covered vents are enemies.
Next week — Software troubleshooting.