TF246017: Team Foundation Server could not connect to the database. Verify that the server that is hosting the database is operational, and that network problems are not blocking communication with the server.
TF400324: Team Foundation services are not available from server [BUILDSRVR]\DefaultCollection.
These errors can occur when TFS is rebooting, when someone using Visual Studio is trying to check in/out items.
For more on the TFS error codes see here (2005)