Yes, I think that Debian probably reformatted the partitions which would have changed their UUID. If that's the case, anything that was on them is gone.
Yes, you should be able to use a live CD (or Debian) to edit and correct /etc/fstab on the Ubuntu root partition.
You will be dependent on Debian's installation of GRUB. If you want to revert to Ubuntu's GRUB, the two commands in Ubuntu are:
but I would probably leave Debian's GRUB on there unless there is a reason to change again.
sudo grub-install /dev/sda