Actually booting from ISO is not the first step to take while troubleshooting servers down issue. Number of options can be tried before that -
Sometimes a simple reboot may resolve the issue.
You can try doing a "fastboot" for the server
Change the default kernel version to the older one or the last stable one
Now , in scenarios where someone removes a kernel related RPM / delete a file like grub.conf or initrd or any system dependent files , then the only solution is booting from iso .
I am attaching a really good comprehensive document that will guide you with boot process and related issues.