db yum_repositories setprop base status enabled
signal-event yum-modify
You probably want to run the command I provided to see if there is anything else unexpected about your yum configuration.
I can only think of two explanations for having the 'base' repository disabled:
1) This server has been continuously updated from SME 7.1, but never had the
yum configuration reset--> in this case, there will be other repositories that should be enabled by default. The basic structure of the SME yum distribution system changed when v7.2 was released. (in 7.1 and earlier, SME Devs migrated, compiled, and distributed many more packages from the SME repositories; starting with 7.2 unmodified Centos packages have been installed and maintained using the default Centos repos)
2) Someone managing the server has been intentionally ([edit] or unintentionally) modifying the yum defaults
--> in this case, there may be totally unexpected yum repositories configured, and you really should check thoroughly. It can be painful to recover from an update that uses the wrong repository.