No Warning Before Self-Rebooting.

Usually before getting restarted,the processor always gives warning i.e Restart Now or Restart Later.
But my computer randomly turns off itself and itself gets restarted.

What is the problem?Why it is happening?

• Check the system logs and see if any message is written regarding a hardware or software problems.
  
• Also, check whether the power-supply is stable, or anything else show evidence of the power momentarily turning off and on.
  
• Check whether any updates are getting downloaded that needs to run causing automatic reboot your computer.

There are many reasons causing your computer to reboot itself.
The main reason is the weak power-supply from the circuit-box of the cabinet.

Usually, when you switch on your computer, first the power is supplied to all the three components(motherboard, hard-disk, cd-drivers) of the system.

If the power-supply is weak, then it may wont reach in any of the components causing the computer to reboot itself.

Some of the Reasons responsible for Computer Self-Booting

Computer virus:

Computer viruses such as the blaster virus are written to reboot your computer without warning. Usually these viruses will make the computer reboot ever 5, 10, 15 or 30 minutes after the computer has booted.

Hardware issue or error:

Any failing hardware component in your computer could cause your computer to unexpectedly generate an error and/or reboot without warning. If you have recently attempted to add a new hardware device, remove that device to make sure it is not causing your issues.

Software issue or error:

Errors generated by software programs can cause a computer to reboot unexpectedly or without warning.
For Windows XP users
Windows XP is designed to automatically reboot each time an error occurs such as a BSoD. Although this may be nice for errors that do not occur often, users who have a re-occurring error may wish to identify the error to troubleshoot it. Below are the steps on how this feature can be disabled and enabled in Windows XP.

1. From the desktop right-click on My Computer.
2. Click the Properties option.
3. In the System Properties window click the Advanced tab.
4. In Advanced click the Settings button under Startup and Recovery.
5. In the Startup and Recovery window uncheck the Automatically restart check box.
6. Click Ok.

Heat related issue:

Many computer today are designed to turn off or automatically reboot if the computer, processor, or other device in your computer gets to hot.