At some point you opened a solution offline, and this caused the associated server to be marked offline. The messagebox you are seeing is due to a bug with opening solutions from Source Control Explorer while the server is marked offline. The bug should be addressed in VS 2008 Service Pack 1.
The fix/workaround for your issue is to mark your server online again by taking a solution online. You should see a Go Online option in Solution Explorer (right-click menu or toolbar button). Use that to take your solution online. After that you should be able to open solutions from Source Control Explorer again without issue.
The fact that taking a solution offline also marks the server offline is catching some of our users offguard.