raid not booting: error: disk not found

[Art Edwards]

My system, a Dell T7500 (8 CPU x86_64), running dual boot between ubuntu
(natty) and CentOS, has suddenly stopped booting. I get:

error: disk not found
grub rescue>

I should point out that the ubuntu side, where grub resides, is RAID 1,
while CentOS is not.
I have booted into a live cd and can mount all of the disks and see all
of the partitions. fsck returns no errors.

Any insight would be greatly appreciated.

Art Edwards