The keys to resolving this issue are:
md1 : active raid1 sdb1[1] sda10 8188864 blocks super 1.2 [2/1] [_U]This is telling you that md1 (which represents the second raid) is supposed to be comprised of 2 partitions - sdb1 and sda10. For some reason sda1 has been dropped from the array, so it is now in a degraded state.
sda1 means the first partition on the first disk the OS recognised. The next step is to find out why this failed, and to fox or replace it.
I would run a command like "hdparm -I /dev/SDA" which should tell the make and model of the disk - so you know which disk you are dealing with. I would then look in the log file for errors relating to /dev/SDA. If you find these, its likely the disk needs to be replaced. If not, it is possible that the signature of on the disk saying its part of the array is missing. In this case it can be readded to the array using the mdadm command. You should also investigate why this happened.
If the disk is failing you need to (a) dump the partition info, (b) degrade md0 - ie remove /dev/sda2 (c) replace /dev/sda (d) partition the new sda with partitions no smaller then on the original sda (e) mark these as type FD = raid partitions (f) reads partitions using mdadm. You can watch the rebuild with "watch cat /proc/mdstat"