did this happen after installation or some time after?
from a previous thread, the software installs in administrator mode, so did you close everything, shut down everything, reboot then restart arcmap?
edit
since Earth is younger than 10.1, perhaps something in the install is not compatible, so check the system requirements for Earth to see if something was updated that isn't good for 10.1 System requirements—ArcGIS Runtime SDK for .NET | ArcGIS for Developers