I loaded up a current application project to implement some updates to the code and all of a sudden my program won't compile.
I've already got versions of this program in use which work just fine.
The issue seems to be with the Microsoft office object library. I have checked my references and it is shown as checked.
Yet VS2013 now cannot find the library. I have removed and re-added the library with no effect.
No other libraries seem to be affected.
I managed to fix it.
I added the DLL files for interop.excel and interop.outlook manually by going to properties --> References --> add --> Browse --> locate the dll library.
I used this MSDN page to figure out where they were and this worked.
Thanks to all who offered help!