Disaster recovery help sought

[MR ZenWiz]

I tried to upgrade my Xubunutu from 12.04 to 14.04 and the system came
up in grub-rescue, unable to find a kernel image.

Being as how there appears to be no help facility in grub-rescue, I
couldn't find a command of any use, so I decided to install from the
DVD instead.

The install appeared to go fine, but when the system rebooted, it came
up with the grub menu, then the screen went blank and stayed blank,
with nothing at all visible.

In partial anticipation of this possibility, I copied everything on /
to a backup directory, but when I tried to restore this, I could not
get grub-install to work - it complained about missing /cow, which I
don't recall ever seeing before.  This was from the "Try Xubuntu"
option from the DVD.

Assuming that I can copy back the files from the backup directory to
the intended root disk, how do I tell the system (grub?) what and
where to boot from?

I used to know this stuff, but 12.04 was stable as a rock for the last
two years, so I'm a bit rusty here.

Thanks.

MR
.