Restore mdadm

mv /etc/mdadm/mdadm.conf /root/

mdadm -Escpartitions > /etc/mdadm/mdadm.conf

mdadm -A /dev/mdX

Once the system is up, compare the output of /usr/share/mdadm/mkconf with the contents of /etc/mdadm/mdadm.conf and update the file with the data from the command output.

When done, run update-initramfs -u and now the system should boot.