It is likely you are seeing this issue because you have "UEFI secure boot enabled" and the sunrpc kernel module is not properly signed with an X.509 certificate that is trusted, or that trusted key/cert is not "installed within the UEFI", or fedora18 might be old enough that it isn't properly outfitted for secure boot (meaning your kernel and/or "shim" may also not be signed and ready for secure boot).
If you can get into the UEFI (think of this as the "new BIOS/CMOS" for new generations of machines, so you are trying to get into the "BIOS" if you are familiar) then try disabling secure boot to get booted into the OS.
At that point, you can determine what is actually happening among the possibilities:
If fedora18 is not properly equipped for secure boot, then you are going to need to leave secure boot disabled or go with a newer version of fedora.
If sunrpc kernel module is already properly signed and the certificate is just not installed into UEFI, look into:
man mokutil
Again, fedora18 might not be "new enough" to have knowledge of secure boot and/or have mokutil or pesign or some of the other utilities that help with UEFI secure boot.
Similar problems will occur if you try to build your own loadable kernel modules and insert them into the running kernel with modprobe or insmod without having signed them.
Here is a good page that can get you started understanding what UEFI secure boot is all about:
http://www.rodsbooks.com/efi-bootloaders/secureboot.html
Welcome to ask.fedora. Fedora 18 reached its End Of Life several years ago and is no longer receiving updates, even for security reasons. Is there a reason that you can't install or use the two supported versions, Fedora 24 or 25?