When you connect to a different team foundation server which has a project with the same name, you may find that your work items suddenly disappeared or you may find new work items. It is also possible that you get authentication errors. Many of these problems are due to the cache that the client keeps.
Delete the cache and most of these problems should go away.
On Windows XP, the cache is at
\documents and settings\username\local settings\application data\Microsoft\Team Foundation\Version\Cache
Close your Team Foundation clients like Visual Studio or Team Explorer, make a copy of this folder elsewhere and then delete the “cache” folder.
Now try again and it should work in most cases.