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.")