I had the exact same issue on 17.10 + XPS 13 + Dell firmware update. I booted in the BIOS setup and manually pointed the EFI to ubuntu's grub's EFI file. That immediately fixed the problem.
(not the parent, but I had the same laptop with the same problem) I reinstalled Fedora. I could have probably recovered the grub bootloader manually, but I wasn't really keen on sitting there for a few hours. All my data was backed up anyway.
It still allowed you to boot from USB? How do you recover if you lose that option (other than sending in your laptop to be reflashed)?