1. Open the Local Group Policy Editor.
2. In the left pane, click on User Configuration, Administrative Templates, and Start Menu and Taskbar.
3. In the right pane, right click on Remove and prevent access to the Shutdown, Restart,Sleep, and Hibernate commands and click on Properties.
4. To Show Shutdown Options -
A) Select (dot) Disabled or Not Configured and click on OK.
or
1. Click on start button.
2. Type gpedit.msc and press enter.
3. Go to User Configuration\Administrative Templates\Start Menu and Taskbar
4. Make sure that the option "Remove and prevent access to the Shut Down, Sleep and hibernate Commands" is either set to Not Configured or Disabled.
5. Close Group Policy editor window.
or this
Running the System File Checker…
1. Click Start, click All Programs, click Accessories, right-click Command Prompt, and select Run as Administrator.
2. Click Continue or supply Administrator credentials if prompted.
3. In the Command Prompt window type the following, and press Enter:
sfc /scannow
4. You should see the following on-screen messages:
Beginning the system scan. This process will take some time.
Beginning verification phase of system scan.
Verification % complete.
or alt + ctrl+ del, shut down and it should come back. Thanks
Edited by KILLER_K - 5/28/12 at 5:19pm