Did it actually not boot or did it seem to hang and the guy resets it after a minute? I ask because my PC had exactly this problem. Ages ago I had a drive die in my system so I pulled it but missed one of the references in /etc/fstab and when I did the initial conversion to systemd it hung and I was about to pull my hear out but instead I left the room to clear my head and came back several minutes later to find my system booted with no explanation as to why the delay.
The systemd update a few weeks ago finally gave me a nice message on console to let me know that one of my fstab entries was timing out so I checked, found the entry and now everything boots faster.