SharePoint Search Service is not working

sharepoint-2007windows-server-2008

I am using SharePoint Server 2007 with collaboration portal template on Windows Server 2008. When I use the following function from Central Administration from Application Management -> search -> Manage Search Service, I met with the following error message, any ideas what is wrong?

The search service is currently offline. Visit the Services on Server page in SharePoint Central Administration to verify whether the service is enabled. This might also be because an indexer move is in progress.

thanks in advance,
George