Well, it's not really unexpected and isn't an error. CentOS 5 went End of Life (EOL) at the end of March and all content for it has been removed from the mirrors. You can amend your /etc/yum.repos.d/CentOS-Base.repo file to point to
http://vault.centos.org/5.11/os/x86_64/ (and /updates/ and /extras/ instead of /os/) but that won't fix the main problem which is that it's no longer supported.
There will be no more updates for CentOS 5. It will become increasingly insecure as time goes on. You should migrate to a supported version ASAP. Given that CentOS 6 hits what's known as "Production Phase 3" on May 10th 2017 and only critical security updates will be released for that, that really means "go CentOS 7".
There's a sticky notice about CentOS 5 EOL on all forums and it's been there for at least a year, warning about EOL in advance. There have been mails sent to the Centos-announce mailing list advertising it too.