If you have replaced a disk in your RAID array, it will need to be configured.
Enter RAID BIOS to configure the new disk. When it is recogonized by the RAID controller, you can use it in the RAID 1 mirror.
Power off the server. Pull off all drives. Pull off power cord(s). Wait a few seconds until all lights turn off. Put in the power cord. Plug in only the healthy disk (possibly in the same place). Power on, hopefully the ESRT bios will recognize the disk as a degraded LV and you'll be able to boot off it and subsequently rebuild.
Another option (in case the controller does not see any disks at all, although they are plugged in) is to run a clear CMOS cycle (J1J1 jumper iirc but please check exactly) without the disks and add them later.
Both options have worked for me. Sometimes only the full power off cycle did the job, sometimes the other was needed. I hope it helps.