Disk failure, on the hardware side, is too broad to cover in any great detail here, but the following basic steps should be followed (of course, as noted, your setup may require otherwise). The scenario here is that one of your disks has just gone "bad." It's beyond recovery.
1. If the disk is hot-swappable, simply remove it. If it isn't, you'll need to schedule downtime and remove the disk then. If the failed disk is your boot disk, you'll already have your downtime ;)
2. Replace the failed disk and restart your machine (and skip to step 6) if your failed disk isn't the boot disk.