How to get an inactive RAID device working again?

Problem:

After booting, my RAID1 device (/dev/md_d0 *) sometimes goes in some funny state and I cannot mount it

# mount /opt
mount: wrong fs type, bad option, bad superblock on /dev/md_d0,
       missing codepage or helper program, or other error
       (could this be the IDE device where you in fact use
       ide-scsi so that sr0 or sda or so is needed?)
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

The RAID device appears to be inactive somehow:
# cat /proc/mdstat
Personalities : [linear]  [multipath]  [raid0]  [raid1]  [raid6]  [raid5]
                [raid4] [raid10]
md_d0 :  inactive sda4 [0] (S)
               241095104 blocks

# mdadm --detail /dev/md_d0
mdadm: md device /dev/md_d0 does not appear to be active.


Resolution:

run,

#mdadm --examine --scan >> /etc/mdadm/mdadm.conf
reboot the machine and you will found /dev/md0 will be active if you run

#cat /proc/mdstat