When we are working on the system it could not restart, but after working we leave 2 minets for system it's automatically restart why?

To give you a good answer you need to answer this things:

 - Is it your own computer?

 - Is it a company computer?

 - Does this computer connected to a Domain?

Answers for your prblem. 

 - If this is your own computer the problem maybe on the OS “Operating System” or a Virus on your system

- If this is a company computer without Domain the problem maybe on the OS “Operating System” or Virus on the system.

 - If this is a company computer connected to a Domain its possible your Administrator set a Group Policy that once your log on to a computer you could not restart your system unless you log out. Or if the computer has been idle for a certain time the computer will restart. This is for security purposes or computer maintenance. If this is not the reason then go back to OS problem and Virus problem.

Hope I helped.

Possible problem software issue, after restart try to observe.