What would be the simplest method of repair; upgrade via disk, single user mode using disk? Thanks.
this page may help
"The kernel does not boot:" section
you should be able to boot of the old kernel that worked but freebsd only keeps one copy so be careful.
from here you will be able to fix what is broken. I guess you did not use the default kernel options and have removed something that is needed by your server
or instead of moving the kernel you could just load the old kernel like so
unload kernel boot /boot/kernel.old/kernel