The first step with things like this is to make sure yum's config has not become confused at some point, do a refresh like this:
yum clean all Then try to run your install or update again.
But, based on the log output, it looks like your remote repository is not available at the given address, hence the 404 message. A 404 is HTTP speak for -
I'm the server responsible for the address you have requested, but sorry what you're asking for is not here.
This at least indicates you don't have a network connectivity problem.
Probably the best thing to do in this case, is look for an alternative mirror address for the repo, or use an alterantive repo entirely.