[CentOS] lost CentOS repositories from yum

Hi,

I was using yum, which was working very well. Then I decided to try
yumex, and installed yumex, which again worked fine. But, after that,
when I came back to use console yum, it started giving error messages.
So, I removed yumex and tried, in vain. Then I removed yum, and
installed it with rpmforge repository.

It works fine, but I only have rpmforge repository. How can I add
CentOS repositories like Addons, Extras, etc back to my yum?

Thanks