Neither of the above solutions worked for me on Visual Studio Community 2017 v15.7.1. Somehow, there was no "Go Online" option in the context menu. I tried registry edit as suggested here, but that only displayed me error that it could not find the binding. What worked for me is rebinding solution to the server from Change Source Control menu.
Go to File->Source Control->Advanced->Change Source Control
and make sure that your solution is binded to your source control. If not (like mine) then click on bind button, it will automatically search online TFS server and rebind your solution to it.