NCR 3430 problems
I have an NCR 3430 MP-RAS SVR4 UNIX System ( NCR UNIX System V/386 Release 4.0 ) and a year ago would normally boot until the part it would ask for a username and a password, but now after it checks everything (RAM, CPUs and Disks) it says " vfs_mountroot:cannot mount root " and it won't accept anything from keyboard (ex: Ctrl-Alt-Del) ( the /root and the /user is installed on separate disks ). If you have any ideas please tell and help ! 
Comments
If the drives aren't always spinning up, there are various tricks that might get them going (tapping, warming, cooling, etc), but you will want to replace those drives.
If they spin up Ok, Then I would "refresh" and test the disks to ensure there are no serious disk errors, and force any weak sectors to re-map. You can do that by dd-ing the hard disk back to itself, or use a "low-level" formatter such as that built in to AHA-2940 BIOS, and then restore the contents.
Once you have verified the hardware as sufficiently functional, you may then need to repair the filesystem.
( found here https://www.freebsd.org/releases/8.4R/announce.html ) and then boot from the NCR, right?
Doing anything with the file system is dangerous until you have verified the integrity of the drives themselves.
Your NCR 3430 would contain unusual hardware and is likely not fully IBM PC compatible. This class of machine would not know how to boot from a CD. And even if it could, FreeBSD is not likely to still support that sort of hardware.
You would boot FreeBSD on a more recent computer (perhaps whatever you are running Linux on), with the SCSI drives connected, and then try to mount the file systems. But I am not familiar with those filesystems or what filesystems FreeBSD currently works with.