If your graphical user interface (be it Xorg or Wayland based) does not start anymore, systemd will get into a restart loop: It will start the graphical interface, that will crash, rinse and repeat.
To avoid this, back in the times of SysV-Init, you would boot to single user
mode by specifying a target runlevel in the bootloader configuration. This does
not work anymore under systemd, but there's an easier way: Booting into the
multi-user target instead of the graphical target!
Edit your kernel line and add the systemd.unit=multi-user.target parameter,
for example:
linux /vmlinuz-linux root=UUID=08dc1e7b-c5a8-4395-ab97-fe5c665943de \
rw cryptdevice=UUID=2fe76075-e3a8-462d-b8aa-861ed138d492:root \
root=/dev/mapper/root quiet acpi_osi=linux systemd.unit=multi-user.target
(Without the newlines of course.)
That's it, now you're booted into a non-graphical system that – in contrast to the classical single-user mode – has full network access!