Failed to successfully launch or connect to a child MSBuild.exe process. Verify that the MSBuild.exe
Quitting and re-opening Visual Studio solved it for me.
Just had the same issue this morning. This error usually appears when your system is running low on memory. If quitting and re-opening VS doesn't do the trick, try closing some of the other applications that are currently running. If all else fails try restarting your system.
If you have tried everything in this post (restart, reboot, etc.) and nothing helps. Try Running VS2012 as Administrator and see if that helps.