Well, you certainly can't expect to find a "cookbook" solution to this problem: you're going to have to deduce what has gone wrong. Let me try to help...
When you say that "two nodes have gone down 'for unknown reasons,'" the first thing to do must be to try to guess what those 'reasons' could be. Did we have a power-hit? Might a disk-drive have failed?
When you see a message like fs_error_blah... it certainly sounds pretty darned plausible that a drive might have gone south for the winter. And so you might need to try booting-up one of these machines from a CD-ROM (such as a handy Knoppix distro) to see if you can make sense of what the disk-drives now contain.
Knoppix, as you may or may not yet know, is a stand-alone Linux distro that can boot entirely from a CD-ROM. It will search for your drives and mount them, if it can, but none of those drives will be "your system drive," which means that you can do what you otherwise could not do: you can diagnose your hard-drives, mount them or dismount them or repair them or what-have-you.
Knoppix is pretty-good at doing diagnostics on its own at startup, but eventually it will drop you to a root-prompt. Start by surfing Google for what they have to say about Knoppix, and about tools like fsck and smartctl.
The first thing I'd do, probably, is to use SMART to diagnose the drive. Hard-drives have on-board diagnostics. When the drive encounters an error, it records it. SMART can find and print-out those logs. If the drive's toast, "hope you got a backup."
The second thing will be to analyze and try to repair the filesystem. Once again, start by surfing Google (and this site).