cannot log in via console, cannot su(1), only as root

[martinko]

hello list,

i was updating sw configuration of my old computer and towards the end i 
noticed i couldn't log in on local console other than root.  i tried 
remote ssh using public keys which worked but i found out su(1) to any 
user except root does not work either.  i've no idea how i could 
possibly manage to "break" my system like this and unfortunately i need 
to fix it by tomorrow.  therefore i would really appreciate your prompt 
hints as what to check etc.

many thanks in advance !!

martin