The solution is offline because its associated Team Foundation Server is offline.

I kept getting the following message in the Output window ,when opening a solution in Visual Studio 2012,

This solution is offline. [Team Foundation Server:
http://server:8080/]
The solution is offline because its associated
Team Foundation Server is offline.

I tried different ways to get rid of this .Searched on the internet and

found different methods bu unfortunately nothing worked for me . 
Whenever I opened one project from the TFS project list , it always
shows offline when I look for the Source control Explorer.From the 

message in the output window ,the solution was offline with TFS.

In TFS once a solution is offline ,it stays offline until you manually

bring it back to online. We can bring a solution  back to online by right

clicking on the solution and select the option  'Go Online'.
This solved my problem . 

Pleasecheck the below link for details:

http://msdn.microsoft.com/en-us/library/bb892960.aspx