Had the same problem and tried all sorts of values for /collection, finally hit the right one and all TFS2012 tf commands started working again.
In Team Explorer in VS right click the server name and select properties, you need the entire URL parameter including http, the port number, everything.
for me that was
http://devserver:8080/tfs/defaultcollection
so the command is just
tf workspaces /collection:"http://devserver:8080/tfs/defaultcollection"