Is the computer plugged in? This sounds simple enough, but you'd be amazed how often a power cord is the source of the trouble. Show me a stone-cold dead PC and I'll find a loose power cord, usually at the computer end. Alternately: The power strip/surge protector has somehow been switched off.

Are you looking at the right cord?  If the computer starts but the monitor doesn't, guess which power cord it is? That, or it's the monitor cable if the monitor turns on but there is no picture.

Plug and replug. And if the network, modem, keyboard, or mouse is on the fritz...until proven otherwise, it is always a cable problem. Turn everything off and unplug and replug all the cords and cables, and many problems will work themselves out.

Have you checked your PC cards? Likewise, check to see if a cable has somehow wiggled one of the add-on cards out of its socket. If any of the cables attached to your PC want to wiggle around despite being firmly screwed in, this is likely your problem. You'll have to open the computer to fix this one.

Is there ink/toner in the printer? It's amazing how a lack thereof can impede your printing efforts.

Are you sure the phone jack works? When you plug a regular phone into your modem line, do you get dial tone? If not check any adapters that may be plugged into the phone jack.

What have you changed recently? While it's true that computers sometimes break down for no reason, most problems have something to do with new hardware or software and occur shortly after installation. I am amazed by how often I ask people what software they've recently added/uninstalled and they can't tell me--until the sixth or seventh time I've asked. Does uninstalling, then reinstalling problem software fix things?

Does removing the hardware clear up the trouble? USB devices are a problem in this regard as the computer may not be able to provide enough power to the device. Trying uninstalling as many USB devices as you can and see if the problem goes away--some devices, for example, don't get enough power from a hub but work fine when connected directly to the computer.

Where are you booting from? If there is a diskette in the drive or a CD is trying to boot your computer, you can get really odd errors--so make sure all the drives are empty.

When in doubt, reboot. Finally, of course you've already rebooted the computer (more than once, if necessary) to see if it solves the problem. You have, haven't you? You'd be amazed how many people stare at a frozen computer waiting for it to come back to life. This sometimes does happen, but after about 10 minutes of waiting, consider a power-down reboot. Likewise, sometimes it takes multiple reboots to make a problem go away.