My Xen DomU’s that I upgraded to 11.4 no longer booted, because the xen-blkfront module is missing from the initramfs. To fix, do this when you get the initramfs prompt:

modprobe xen-blkfront
exit

Then after it’s booted:

root@ubuntu:~# echo "xen-blkfront" >> /etc/initramfs-tools/modules
root@ubuntu:~# update-initramfs -u

Source.

Also look at this post.