Description: My TFS(Team Foundation Server) is connecting to source code server, however, the code in server can NOT be checked-out (so you can NOT see the red “checkmark”) when I try to edit and do some changes.
Cause: The state of the code in your client is offline when TFS misses connection to server.
Solution: Just set the state of the code in your client to online.
Pay some attention to reconnecting TFS
原文：http://www.cnblogs.com/freecodeX/p/4350567.html