Sometimes Visual Studio 2012 goes slightly mad and displays an error message (did not take screenshot, sorry) that it has encountered an exception that might be caused by an extension, and mentions the Activity Log file.
This file is normally located at %appdata%\Microsoft\VisualStudio\11.0\ (where %appdata% is normally C:\Users\<yourUserName>\AppData\Roaming) and is called ActivityLog.xml – one can check the problem in more details (TIP: there is the corresponding xsl file, so the xml file opened in IE will be nicely formatted).
Usually the exception message stored in the ActivityLog.xml will clearly point to the offending extension/plugin.
If it does not, the deleting the %localappdata%\Microsoft\VisualStudio\11.0\ComponentModelCache\Microsoft.VisualStudio.Default.cache file (where %localappdata% is normally C:\Users\<yourUserName>\AppData\Local) might solve the problem. It is safe to delete this file as it will be rebuild when Visual Studio is starting. And, of course, Visual Studio should be closed when you are trying to delete this file.
If even then the problem persists, then pray to god(s) – there is no help for you! :)