Mdadm array is inactive, how to fix?

Question

I have a RAID5 array configured with MDADM in Ubuntu 24.04 LTS. The array consists of five 8TB drives, and there have been no changes to it recently.

Today I noticed that the array is not accessible.

• mdadm --detail /dev/md0 shows that the array is Inactive:

  $ mdadm --detail /dev/md0
  /dev/md0:
             Version : 1.2
          Raid Level : raid5
       Total Devices : 5
         Persistence : Superblock is persistent
               State : inactive
     Working Devices : 5
  
                Name : Europa:0  (local to host Europa)
                UUID : 62595935:e04505fc:3e79426a:40326185
              Events : 76498
  
      Number   Major   Minor   RaidDevice
  
         -       8        1        -        /dev/sda1
         -       8       81        -        /dev/sdf1
         -       8       65        -        /dev/sde1
         -       8       49        -        /dev/sdd1
         -       8       33        -        /dev/sdc1
  

• Using mdadm --examine of each drive, I find that they all say the state is Clean, and 4 of them show the array state is AAAAA (all drives are active), but one of them (sda1) shows the array state is ....A

• Using cat /proc/mdstat I find that ONLY device sda1 appears!

  $ sudo cat /proc/mdstat
  Personalities : [raid1] [raid0] [raid6] [raid5] [raid4] [raid10] 
  md1 : active raid1 sdg1[1] sdh1[0]
        2930132992 blocks super 1.2 [2/2] [UU]
        bitmap: 0/22 pages [0KB], 65536KB chunk
  
  md0 : inactive sda1[4]
        7813893632 blocks super 1.2
  

• By looking at mdadm --examine for the event count and update time, I can see that sda1 has slightly more events and a more recent update time than the other drives:

  $ mdadm --examine /dev/sda1 /dev/sdc1 /dev/sdd1 /dev/sde1 /dev/sdf1 | egrep 'Event|/dev/sd'
  /dev/sda1:
           Events : 76498
  /dev/sdc1:
           Events : 76490
  /dev/sdd1:
           Events : 76490
  /dev/sde1:
           Events : 76490
  /dev/sdf1:
           Events : 76490
  

  and

  $ mdadm --examine /dev/sda1 /dev/sdc1 /dev/sdd1 /dev/sde1 /dev/sdf1 | egrep 'Update Time|/dev/sd'
  /dev/sda1:
      Update Time : Mon Jan 13 14:51:59 2025
  /dev/sdc1:
      Update Time : Mon Jan 13 05:03:20 2025
  /dev/sdd1:
      Update Time : Mon Jan 13 05:03:20 2025
  /dev/sde1:
      Update Time : Mon Jan 13 05:03:20 2025
  /dev/sdf1:
      Update Time : Mon Jan 13 05:03:20 2025
  

So my question is how to interpret this?

When searching online I found most people reporting the inverse: one drive has fewer events than the others, and one drive has an earlier Update Time than the others. I can't imagine that 4 drives went bad at exactly the same time, especially since there was no power failure or anything else that might explain a widespread hardware issue.

So what does this mean, and how do I recover the array?