freebsd 6.4 can't load kernel after upgrade

[oscar Seo]

I'm a beginner in freebsd.
my machine consists of freebsd-6.4 + i386 bootstrap loader,+ windowmaker
after upgrade freebsd-6.4 using sysinstall then reboot the system,
I got an error message as follows
+++++++++++++++++++++++++++++++++++++++
Loading /boot/defaults/loader.conf
Unable to load a kernel!
/
can't load 'kernel'

Type '?' for a list of commands, 'help' for more detailed help.
OK _
+++++++++++++++++++++++++++++++++++++++

so I decided to reinstall freebsd-6.4 but I can't boot and re-install
freebsd using CD-rom.
what shall I do boot my system using installed freebsd or live-CD ?
Thanks...