sometimes it happens that a connection to the DFS server from the DesignStudio/InformationStudio is lost for network reasons.
You can re-connect to the DFS server by going into tools/options/DataFlowService and click ok.
But this can only be done when there are no open plans.
As long as the connections to the repository database is there, it should be possible to re-connect to the DFS server also when plans are open.