It might be a coincidence that X was not comming up after the last dist-upgrade I did on 1004 cause I also added some hardware recently but I thought I'd post it here in case someone else has the same issue. I got a warning in Xorg that nvidia failed to initialize the graphics device in /var/log/Xorg.0.log
(EE) Oct 14 07:08:42 NVIDIA(0): Failed to initialize the NVIDIA GPU at PCI:3:0:0. Please
(EE) Oct 14 07:08:42 NVIDIA(0): check your system's kernel log for additional error
(EE) Oct 14 07:08:42 NVIDIA(0): messages and refer to Chapter 8: Common Problems in the
(EE) Oct 14 07:08:42 NVIDIA(0): README for additional information.
(EE) Oct 14 07:08:42 NVIDIA(0): Failed to initialize the NVIDIA graphics device!
Eventually I created a custom boot entry for grub in /etc/grub.d/40_custom by adding to the end of the linux line the option (just copy the existing one from /boot/grub/grub.cfg):
then running update-grub and checking in the file /boot/grub/grub.cfg which entry number the new entry and adding that number to /etc/default/grub as the GRUB_DEFAULT (I also set the GRUB_HIDDEN_TIMEOUT=2 and GRUB_HIDDEN_TIMEOUT_QUIET=false)
then ran another update-grub and rebooted and I got X again.