Originally Posted by jagdip
What could have changed with installing new kernel?
Did you do something more involved than a standard update that included a new kernel image?
Things that come to mind include: missing a necessary module or part of the initram image, changes to PAM, /etc/passwd, /etc/group/, or (most importantly) /etc/shadow. Unless you somehow overwrote these files or replaced them with a package default, you should not have lost user information from an upgrade.