Following up on my earlier... if I remember correctly.
When grub installs it uses references to physical drives. The first drive is hd0 and the first partition hd0,1 etc.
If your 1st drive fails do not just disconnect it and expect the server to boot as it most likely won't.... the 2nd drive will become the 1st drive and grub can then get in a muddle with drive numbering.
Either leave the old drive connected and hopefully the BIOS will at least know there is a drive there and number them accordingly and it will run with the array degraded, or the better option is to replace it with a new drive.
(I got in a knot like this myself once)
If it still won't boot with either the old drive in place or a new drive in place then you as per JPs suggestion you will need to use the repair option from the install disk.
- booting arch from a USB, then fdisk -l shows all the mountable raid partitions (ext3), if the working and failing drives are connected.
So an emergency boot disk can read all the partitions on both drives?
So exactly what error did you have on the first failed boot? Just a flashing cursor, or had you disconnected the first drive? Any other errors at all - eg smart failures, bios errors at startup etc?
I'd suggest not to touch mdadm yet - that could just exacerbate the issue. If you can get it to boot on one drive, it will run as a degraded array - not good but will allow you to work, add another drive to the array etc.