Please follow the following steps
If you are using Windows Vista or Windows 7:
1a. In the Start menu, type Services in the search field,
and press enter.
1b. You may need to click continue if you are prompted to verify
this action.
If you are using Windows XP Service Pack 2 or 3:
1a. Go to Start | Control Panel | Performance and Maintainance
| Administrative Tools.
1b. Open Component Services
1c. Double-click on Services (Local)
2. Locate the Security Center service.

3. Right-click the service, and select Automatic (if there is
such an option), then Start.
You may receive a error message after that. If that occurs,
please post it here. If there is no error and the service is
started, run Security Centre again to see if it now
works.
To be able to provide more detailed information and steps, we
will at least need to know what operating system you are
using.