Most administrators configure group policy to use a domain internal Windows Update Server (WSUS), so if you take your domain member computer off the domain network (assume notebook or virtual machine image), the system won’t be updated reporting error: cannot check for updates or cannot download updates.
“You receive updates:” is configured as “Managed by your system administrator” and “Check online for updates from Microsoft Update” is disabled/grayed out.
The reason is mostly configured group policy, pointing to use a domain update server (WSUS server) as update source.
You can revert your box to get updates online instead of looking for (temporarily?) unavailable update server.
Open regedit on the box and change following registry value
and retry check for updates:
Refer also Microsoft KB http://support.microsoft.com/kb/328010 and enjoy!