I don't understand what you mean by "installed Linux system, not on fresh system."
Debian has an option to boot in recovery mode (single option to the kernel). If this is available, it should show up in the grub menu. If it is not available in the menu, you could always add it. This is how it looks on my system.
menuentry "Debian GNU/Linux, with Linux 2.6.32-5-vserver-686-bigmem (recovery mode) (on /dev/mapper/olddebian-root)" --class gnu-linux --class gnu --class os {
insmod part_msdos
insmod ext2
set root='(hd3,msdos5)'
search --no-floppy --fs-uuid --set=root c8231ae9-c018-45ba-8812-9170eb82449a
linux /vmlinuz-2.6.32-5-vserver-686-bigmem root=/dev/mapper/olddebian-root ro single
initrd /initrd.img-2.6.32-5-vserver-686-bigmem
The relevant line is
linux /vmlinuz-2.6.32-5-vserver-686-bigmem root=/dev/mapper/olddebian-root ro single
EDIT: If you want to a live CD to boot up in text mode, check out Grml. It has an option for lots of things, including booting in text mode. Other live CDs do as well, including Knoppix, but Grml is particularly flexible and full-featured. Note, to boot Knoppix in text mode, enter knoppix 2 at the prompt.