You can see that in the mdstat output. If you compare the working array md0:
md0 : active (auto-read-only) raid1 sda1 sdb1
with e.g. the broken md1:
md1 : active raid1 sda2
then you see that the first line lists all devices that are part of the array. in md1, the array consist only of the sda2 device, the sdb2 device is missing.
Before you start to repair the array, please ensure that you have a backup of all data of that server. It might always happen that data gets lost.
Then try to readd the sdb2 device to md1:
mdadm --manage /dev/md1 --add /dev/sdb2
and the sdc2 device to md2:
mdadm --manage /dev/md2 --add /dev/sdb3
If this worked, you should see with mdstats that the devices get resynced now: