If you have multiple versions and side by side installation of AutoCAD MEP and AutoCAD Architecture 2012 and/or 2013 and have needed to or you have removed any of the service packs you may run into more issues with your software than you wanted.
If you have removed any of the 2012 service pack(s) and are now having issues launching on of your 2012 software it can be due to the removal of another service pack. You will need to un-install and reinstall both the 2012 software and its service packs.
Autodesk has added this in the notes of the service pack so that you are aware of this issue. Here are the service packs for AutoCAD MEP 2012 and AutoCAD Architecture 2012.