Hi DHANUSH ,
Check weather that your computer Power button is Strucked while pressing or not.
If power button is ok then try to unplug your keyboard and try to On your computer. i think the problem will be solved.
If not Remove any previously add hardware which can cause the problem. or unplug your network wire or WIFI Dongle (remove internet Connection) from your computer so that Virus in your computer cannot communicate with outer world . Install Good Antivirus and scan the Computer