Hi, I have a problem on the start menu. I can’t find the button that gives option to
shutdown, restart and hibernate. Is there someone tells me that how to get the button back?

You can right click on the taskbar and select properties. In Start Menu tab, open the drop-down menu aside the Power button action and select Shut-down and click OK.

There is another trick:
1. Open the all users, specific users or groups, or all users except administrators Local Group Policy Editor for how you want this policy applied.
2. In the left pane, click on to expand User Configuration, Administrative Templates, and Start Menu and Taskbar.
3. In the right pane, double click on Remove and prevent access to the Shut Down, Restart, Sleep, and Hibernate commands.
4. Select (dot) either Not Configured or Disabled.
5. Close the Local Group Policy Editor window.