NVIDIA Drivers apper to break yum repository

[TrevorH]

What do you get from rpm -qa ? Is that a very very short list?

[stuartw]

I have re-installed Centos 7 from DVD. I will try executing some of the commands above before and after each step, from the initial update following the fresh install.
There is something else that I have failed to mention that, having thought about it, may be the problem.
I have (among others) /mnt/home. This resides on another file system, which is part of the ultimate plan and very handy when you're re-installing for a living. Therein resides the NVIDA binary that I have run without moving it to the / file system. If the installer assumes a path, it may well hammer something as a side effect. In light of this I am going to run the installer on a fresh installation, after the first update, all with multi-user.target. If the same error occurs, I will install again and try running the installer from a directory in the / file system (e.g. after copying the installer).

[TrevorH]

If you had run rpm -qa before reinstalling then we would have had a better idea of what was wrong. I am suspecting that your entire rpm database had been wiped out or corrupted so it didn't have any packages listed in it.

[stuartw]

After reinstalling, I booted with multi-user.target and ran a yum update from the command line. Following that, I mounted the filesystem containing the NVIDIA installer and as before, ran it in place. Rebooted and yum still works. Installed alacarte and eclipse. Yum still works.
I am at a total loss to explain what happened with three previous installs that produced the error. I would really like to know what happened but need to get on with things. Thanks to all who contributed.