A computer that won't start can be both alarming and infuriating, especially if you're on a deadline or you haven't been backing up all your data (like we all tend to do). Don't panic, though-this problem often has a simple solution.
If your desktop computer does nothing at all when you try to start it, first check that the power cable is securely plugged in at the back of the computer as well as into a working power outlet. It's amazing how often this is the case.
If you're starting from a button on the keyboard, make sure the keyboard is connected to the computer.
If the computer is plugged into a surge protector, see if the surge protector has a reset switch that you can push. (If it doesn't have a reset switch and the problem is with the surge protector, it will need to be replaced.) Try plugging a lamp or other device into one of the surge protector's outlets to make sure it's working.
If you can hear the computer's fan or hard-disk drive, or if you can see indicator lights on the main unit but the monitor stays dark, make sure that the monitor is connected to a working power source, that it's turned on and that it's securely connected to the computer via the video cable.
If the monitor and computer have power but the computer displays a "Non-system disk or disk error" message, check to make sure you didn't leave a disk in the A: drive. If you did, eject it and restart the computer.
If the operating system still won't start even though the computer and monitor have power, try restarting with a Windows CD-ROM in the CD-ROM drive or, if you've got a Macintosh, with a Mac OS disc in the CD-ROM drive. (You can start a Mac from the CDROM drive by holding down the C key while starting up.)
If the operating system then starts, your problem is probably with the startup hard drive or with the operating system installed on it. If you can't fix the drive, you'll have to replace it. (See also How to Fix Bad Sectors on a Hard Disk and How to Recover Lost Files.)
If you've got a laptop computer that suddenly refuses to start up, it's possible that the battery drained too low while the computer was in sleep mode. Try plugging in the power cord to see if the laptop will wake up. Always make sure all your cables are plugged in securely, that usually solves the problem over 75% of the time.