if you have lost RAID in ubuntu

do

df
to see all mounted devices. If you cant see your mounted RAID, check:

cat /proc/mdstat
cat /sys/block/md0/md/array_state

cat /sys/block/md5_d/md/array_state

to check status use

mdadm -D /dev/md0
mdadm -D /dev/md_d5

check conf file, if its empty, add your own entry, as it highlighted below to  to /etc/mdadm/mdadm.conf. To find out your device UUID use blkid

# mdadm.conf
#
# Please refer to mdadm.conf(5) for information about this file.
#

# by default, scan all partitions (/proc/partitions) for MD superblocks.
# alternatively, specify devices to scan, using wildcards if desired.
DEVICE partitions

# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes

# automatically tag new arrays as belonging to the local system
HOMEHOST 

# instruct the monitoring daemon where to send mail alerts
MAILADDR root

# definitions of existing MD arrays
ARRAY /dev/md0 level=raid1 num-devices=4 UUID=f6a9dc91:113e2dcc:e98f006c:315aa4a8

# This file was auto-generated on Tue, 29 Dec 2009 03:16:26 +0800
# by mkconf $Id$

ARRAY /dev/md_d5 level=raid5 num-devices=4 UUID=815a9f2b:ed3c8759:f827a6be:9a5128ee