Hi Ketaki,
Is there any specific time for system to go shutdown. If not then try to notice if any particular software or application is causing this problem when starting.
Does system heat up and shuts down.
Try this:
Boot your system in BIOS, keep it on for almost 3-4 hours. In case if system shuts down then it indicates that your system has Hardware problem.
And if does not shutdown then you should do a clean install of OS which will resolve this error.
Regards,
Steve