Windows tech support: Getting your PC to start
Your PC won't boot. Is the issue loose cables, bumped switches, dead parts? Use this guide to find and fix the problem.
"Why won't my computer start up?" You've probably had that problem or have tried to solve it for someone else. If you know your way around a computer, people come to you with their tech-support questions. After all, going through official support channels can be very frustrating and time-consuming. Why not ask someone you know, whose knowledge you trust? We're building a series of crib sheets on common support questions to help you (or the people quizzing you), starting with the most basic issue: figuring out why Windows won't boot.
We'll assume two things: (1) This is a computer that successfully turned on before, within the last few days, rather than a new PC fresh out of the box or one that's been collecting dust in a closet. (2) We'll also assume that this is not a PC that you built yourself, because the flowchart of potential startup problems in that case gets pretty involved. Many startup problems require replacing internal parts, so there isn't a simple solution for every scenario. But you'll at least have a better idea of when you need a gearhead to fix things.
Cnet.com - Windows Tech Support