Remount it. If /etc/fstab is correct, you can simply type:
mount -n -o remount /
If /etc/fstab is wrong, you must give the device name and possibly the
type, too: e.g.
mount -n -o remount -t ext2 /dev/hda2 /
To understand how you got into this state, see, (“EXT2-fs: warning:
mounting unchecked file system.”)