Solution for Visual Studio 2012 & TFS Problem: "The solution was offline during its previous session and will remain offline."
Here are the steps to solve this problem; Go to registry editor. Go to Computer\HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\11.0\TeamFoundation\Instances\xxx\Collections\XXX Modify Offline DWORD value to 0. Start your Visual Studio Project again and select OK button to connect to TFS server.