freebsd dead after attempt on upgrade from 6.2 to 6.3

[Deian Popov]

Hello,

I'm pretty stuck with the death of my bsd machine after attempt on upgrade
from 6.2 to 6.3. Some time ago, I successfully upgraded from 6.1 to
6.2following the steps in the manual

1) cvsup source to specific release
2) buildworld
3) buildkernel
4) installkernel
5) boot
6) mergemaster -p
7) installworld
8) mergemaster
9) boot

Now, after completing steps 1 to 5 the system won't boot and stucks at the
following prompt

bsd: Trying to mount root from ufs:/dev/ad1s1a
bsd: mountroot>

the prompt has "?" command which shows available boot devices. The only
device present there is my floppy (fd0)

I can choose to boot to loader prompt but have no idea how to recorver, and
more interesting, to figure out why that failure occurred?

Any help is appreciated.