This problem is very easy to solve it . First , if your computer CPU is very hot , the computer will turn off and turn on automatically . Second , if you have missed to plug in your hard disk , ram , or other things tht belongs to the computer , it can't open the computer . Finally , if your computer have virus , it can't open it too .
If you want to check out the problem , you can turn on the computer again , and if you can hear some sound like " Beab " , it means that your have missed something . If not ,
i think that your CPU is too hot , youcan cool down it for a minute , and turn on the computer , I think it will successful to start the window Vista .If you have problem , you can ask it again , and I will answer you . Hope that you can start it , and the next things is , which Vista do you use ? I use Home Basic so you can ask me more about this new operation system . Thank you !