CentOS

Our CentOS 6.6 system will not boot after a recent update, which seems to have erased many system files. It hangs at the splash screen or "dracut: Switching root" in verbose mode. We have found some discussion of these symptoms on the internet, but nothing has worked for us. We have managed to retrieve our data, but we would like to restore the system. Any suggestions? Thanks.

Boot an older kernel from the grub menu.

If that doesn't work then you need to provide a lot more information about what you did, what was updated, what you think is "missing" etc.