FUBAR on an upgrade - need some help

[Kurt Buff]

I have an old Acer AspireOne netbook that's been running 10.1-RELEASE.

Yesterday I upgraded it using freebsd-update to 10.2 and then to 10.3,
and it went fine.

Then I upgraded it to 11.0-RELEASE, and it failed during boot, saying
it wanted to boot from ad4s1a, but couldn't find it.

I have managed to get it into single user, and have run "df-h" and
"gpart show", which don't agree at all. Output:

# df -h
Filesystem     Size     Used     Avail   Capacity   Mounted on
/dev/ad4s1a    140G   36G        93G        28%    /
devfs               1.0k    1.0k          0B      100%   /dev

#gpart show
=>         63   312581745   ada0   MBR    (149GB)
             63   312581745        1   freebsd      [active]     (149GB

=>           0   312581745   ada0s1    BSD     (149GB)
               0   8388608               2    freebsd-swap    (4.0GB)
    8388608    304193137           1    freebsd-ufs      (145GB)

# cat /etc/fstab
# Device            Mountpoint      FStype     Options      Dump      Pass#
/dev/ad4s1b       none               swap         sw             0            0
/dev/ad4s1a       /                     ufs            rw
1            1


I can't seem to use vi to modify fstab.

I'm a bit lost on how to recover from this. Can anyone give me a clue?

Thanks,

Kurt