It appens twice to me. You open the project proeprly. It ask you your user name password for Visual Source Safe (VSS), You check out a doc, work on it. And suddently, there is not more Chech-in button availble in the menu.
-Your project were disconnected from source safe.
Reconnect them… From VS, File -> Source Control -> change Source Control…
Than… The columns with the “Disconected” Header but have unchecked check boxe(s).
You just need to ick them all and click OK.
You buttons will be back on. (You may need to logon to your source safe again though).