If you’re in single-user mode, you’re usually fighting an emergency. An you often need to “undo” some action you took on the filesystem (such as changing a config file) that forced you into emergency mode. Single-user mode mounts the root partition read-only by default. To change a config file, you must first allow read-write on the root partition. Issue this command:
# mount -n -o remount /
This will remount the root filesystem without writing into the /etc/mtab file (which normally happens during a mount operation).