If it’s LILO, just type “linux single” and that should do it (assuming that “linux” is the lilo label).
If GRUB, hit ‘e”, then select the “kernel” line, hit “e” again, and add ” single” (or just ” 1″) to the end of the line. Press ENTER, and then “b” to boot. (More modern grub uses “a” to append to the boot line)
e.g.
kernel /boot/vmlinuz-2.2.17 root=/dev/hda8 video=matrox:vesa:261 single