HELP: Linux 2.6 system suddenly fails to boot

I am ripping my hair out with a linux 2.6 system that has worked reliably for a
number of years, but which now this morning suddenly
failed to boot.

What happens is that the boot sequence gets to the point where it does swapon,
and then can't find the swap partition. The
subsequent attempt to mount the primary partition readonly appears to work, but
when it tries to go read/write, it fails with the
error:

	fsck.ext2: No such file or directory while trying to open /dev/sda3

It then prints out a message about the superblock possibly being corrupt and
suggesting I run e2fsck against it. Iit doesn't drop
into single-user mode; it just processes a few more boot script instructions and
hangs...apparently forever.

The really weird thing is this: I keep a backup Slackware partition on this same
computer, and when I boot into it everything works
fine. More importantly, I can mount and browse the partitions I normally use
(which are /dev/sda3 and /dev/sda4 in my normal boot
sequence, but are /dev/hdc3 and /dev/hdc4 when running under Slackware). 

Most importantly of all, e2fsck reports no errors of any kind on the primary
boot partition, not even bad blocks (I did the
non-destructive read/write test). I ran it against those partitions from within
Slackware.

What the heck is going on?? How can a partition have a clean bill of health when
run under one distribution and fail to mount when
booted under a different distribution on the same hardware??

FYI, my primary distro is LinuxFromScratch.

Any help would be greatly appreciated as I am totally dead in the water here,
and am facing having to rebuild my entire system for
no reason that I can see.