I've seen that symptom before over the years with Linux and UNIX. In
every case it was due to information in the /etc/passwd file re:

1. specified shell is not available (either because it's been deleted
or the partition on which it exists hasn't been mounted), and/or

2. home (i.e., login) directory is not available (either because it's
been deleted or the partition isn't mounted).

Since you cannot login as yourself (or as root due to Ubuntu's policy),
the first thing to do is reboot into single-user mode from the grub menu
and see what's in /etc/passwd at the moment and whether all the info in
that file for your login username points to valid directories and files.

"Something" is going wacky with 8.10; I've read several identical posts
in Usenet's comp.os.linux.* newsgroups and haven't read of any general
solution (yet).

My guess is that something that's been installed after the distro
install is altering "something" on the system due to, perhaps, a race
condition between several processes.

If /home is on its own partition and that partition isn't mounted (for
some reason), the symptoms will be identical to what you report.