Occasionally Visual Studio will balk at copying or pasting text in the edit windows. I recall seeing this problem in Visual Studio 2010 a few times. I was surprised to see the issue in Visual Studio 2012.
The scenario: Select text in any edit window and choose Copy or Cut. Attempt to paste the text. The Paste command doesn’t work, and the paste menus are disabled. It appears that the data is not in the clipboard. Attempts to paste in other applications (notepad, word) also fail.
Remote Desktop is the Culprit
Do you have an active Remote Desktop connection open on your computer? That’s the problem, at least it is on my computers. Remote Desktop integrates with the client clipboard. I’m not sure why that would cause problems for Visual Studio 2012, but closing the connection fixes the problem.