Since you referenced SAM, I am assuming you have HPUX. Is your problem with /usr or accessing the users on your system?
If it is the /usr filesystem read on. If it is your actual users on the box, that is a totally different problem.
If you have a corrupted filesystem, I would suggest going to single user and peforming an fsck on that unmounted filesystem. Then boot to multi and watch the flags on the filesystem during the boot process to see if the filesystem is "clean" or "dirty".
Also, I would try to restore /usr from a backup before wiping out your OS and reinstalling.
As a last resort, I would reinstall, but that is always my last option.
My brain is your brain...