To understand how you got into this state, see EXT2-fs: warning: mounting unchecked file system.
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 /