This gist enumerates a process which worked for me to repair an installation of EndeavourOS with full-disk encryption when it is unable to boot. It is also available as a blog post on my website.

Specifically, this set of steps fixed the boot process on a HP-Envy laptop running EndeavourOS with an ext4 file system. The issue normally occurs after an interrupted update using pacman -Syu, which then causes the system to be unable to boot after the next restart (showing only "boot to firmware interface" in the boot menu).