Boot in a live usb (even the Qubes OS installer, enter TTY with CTRL+ALT+F2) and deal with the qube in question from there.
# If your QubesOS installation is encrypted:
$ cryptsetup open /dev/<device> luks-qubes
# Find the name of the qube of interest and mount it
$ lsblk
# Grep can help you locate the exact qube
$ lsblk | grep <my-qube>
$ mount /dev/mapper/qubes_dom0-vm--<my-qube>--private /mnt
# Do what you need to do then unmount
$ umount /mnt
$ vgchange -a n
$ cryptsetup close luks-qubes
(you may need to run the above as root)
Done.