Look in one of these places, then when you find it add the smb passwd file = option to smb.conf in case you forget
/etc/smbpasswd
/etc/samba/smbpasswd
/usr/local/samba/lib/smbpasswd
/usr/local/samba/private/smbpasswd
(if your samba isn't in usr/local, then find it and check for a private or lib subdir)
Can you tell I've had the same problem? Also don't be fooled by symlinks and size 0 files!