Something suggests that it can not see part of the harddisk anymore, is my first guess.
What I would do:
I would have a go at it with fsck, see if you can read the partition information.
(but do some research on fsck first, make sure you understand how it works.)
then:
disconnect your present scsi disk.
get a another scsidisk, and install sme on that one.
(in order to make sure that all the hardware is ok.)
once you have a running system, mount the original drive as dev/hdx
once mounted have a go at it with fsck..
but..
if you really have important data on the disk, get an expert to help you, because once if you make mistakes, it's gone forever..
good luck