For a standard team site (STS#0) the GlobalNodes SPNavigationNodesCollection will contain three SPNavigationNode objects.

Home, Quick Launch and SharePoint Top Navigation Bar, these each have Ids of 1000, 1002 and 1025 respectively.

The problem occurs if you delete Quick Launch or SharePoint Top Navigation nodes. Even if you then add them back in a new ID is generated and SPWeb.Navigation.TopNavigationBar looks for child nodes under a node with an Id of 1002 (This is hard coded).

Through the API this means that SPWeb.Navigation.TopNavigationBar will always be null and through the SharePoint UI an error will be returned to the user (“Value does not exist in that range”).

The only ways I have found to fix this is to delete and re-create the site or (Microsoft please close your ears) get the site id and web id and query the NavNodes table in the content database the site collection sits in and add the following records

SiteId WebId Eid EidParent NumChildren RankChild ElementType Url DocId Name DateLastModified NodeMetaInfo NonNavPage NavSequence ChildOfSequence
C803FD13-27FA-475F-909C-2CE5B2048E1B A31E8093-1505-4E1C-B7CC-04D6B952A33F 1000 0 0 0 0 NULL A31414EE-953B-44CC-A37E-4D5D643B002A Home 2008-04-04 11:28:21.000 NULL False False False
C803FD13-27FA-475F-909C-2CE5B2048E1B A31E8093-1505-4E1C-B7CC-04D6B952A33F 1002 0 0 2 1 NULL NULL SharePoint Top Navigation 2008-04-04 11:28:21.000 NULL False True False
C803FD13-27FA-475F-909C-2CE5B2048E1B A31E8093-1505-4E1C-B7CC-04D6B952A33F 1025 0 0 1 1 NULL NULL Quick Launch 2008-04-04 11:28:21.000 NULL True True False