Need Some More Details to answer thi question.
Is it a Laptop or Desktop, what brand (HP, Tohiba, Dell etc).
1.Operati ng System (XP, Vista , Seven)
2.is it Overheated (may be caused ddue fan is not working, Hardwar issue)
3.Start your system in BIOS and Leave it for few minutes ,see if it turn off, its an hardware issue, contact service provider, or its a software issue, try restoring , factory restore(if you are ready to loose data).or re-installation of OS may help.
Is it a Laptop or Desktop, what brand (HP, Tohiba, Dell etc).
1.Operati ng System (XP, Vista , Seven)
2.is it Overheated (may be caused ddue fan is not working, Hardwar issue)
3.Start your system in BIOS and Leave it for few minutes ,see if it turn off, its an hardware issue, contact service provider, or its a software issue, try restoring , factory restore(if you are ready to loose data).or re-installation of OS may help.