Software conflict
Hardware conflict
Heat issues
Virus
OS (Operating System) Issue
Software Conflict
Uninstall the software that causes the restarting problem.
Try rebooting your computer in safe mode, then do the following.
From the desktop right-click on My Computer.
Click the Properties option.
In the System Properties window click the Advanced tab.
In Advanced click the Settings button under Startup and Recovery.
In the Startup and Recovery window uncheck the Automatically restart check box.
Click Ok.
Hardware Conflict
Failing hardware may cause the computer to restart.
Use device manager to check for any problems with hardware.
To do this, right click My computer > Properties > Hardware tab > Device manager
Check whether any new drivers has been installed recently and rollback the changes.
Viruses
Viruses may cause the computer to auto restart after a certain time after booting.
Use a good antivirus system and run a scan in safe mode.
Heat issues
Overheating can cause the computer to shut down / restart to prevent it from reaching a high temperature. Check that the power supply sends power to the fans and make sure your computer is well vendilated.
OS Issues
The OS might have issues which causes the computer to restart by itself.
To solve this, either Repair your OS if all else fails or reformat it.