[CentOS] (yum) Error: Cannot retrieve repository metadata (repomd.xml)....

This error:

Error: Cannot retrieve repository metadata (repomd.xml) for repository: 
debug. Please verify its path and try again

occurs every time I run yum (and is where execution stops).  Why? and/or 
What can be done to fix yum so I can use it again?


tnx.

-- 
War is a failure of the imagination.
         --William Blake