I had the same problem.....check you have not got the 'caps lock'on.....I know
it sound improbable but it happened to me....

It's not the caps lock. It doesn't act like caps
lock fault. It goes beyond that and acts like it tryiong to login and goes back
to the login screen.

Do you know for sure which login screen it is? There is one for when you
are logged out and one for when you have locked the screensaver. What you
describe sounds similar to a bug in the gnome screensaver. The solution
there is to kill gnomescrenesaver from alternate tty after boot. I'm not
even sure if you ate running gnome, but thought I would mention this just
in case...

I was in Ubuntu (Unity 3D) 11:10. I was using gnome tweak tool to change my
themes. Suddenly I got kicked out to the Login screen. Not the lock screen.
When I tried to login it blanked the screen for a moment (complete black screen)
as if it was going to login and went back to the login screen. There is some
text that flashes after the screen goes black but it is too fast to read even
part of it. It's white text on black like command line in the terminal.

I have tried logging back in under Unity (3D), Unity 2D, Gnome classic (no
effects), Gnome classic, and Gnome 3. I get the same results each time. I used
the guest account, created an admin level user using the password of the account
that I can't login to, and logged in with no problem under the new account.
Everything is fully functional under the guest and the new account including
gnome tweak tool. So the guest account and the new account are unaffected.

Thanks to everyone for all the suggestions and help. It's greatly appreciated.

If the new account is working fine then perhaps the best way is to
copy data from the non-working account and then delete it. Since it
won't accept the password you'll need to do it from root but you
should be able to just use terminal and sudo nautilus

First things first, have you tried shutting down and starting up
again. It may be that whatever you were tweaking has locked up and is
preventing the session re-loading even though the password is actually
correct.

I've gone all the way to a complete shutdown and a cold start. I can
login using guest and I've created a new user and I can login under that. Is
there a way to reset the affected user?

Have you tried logging in with Unity 2D? (You can choose it from the
settings cog icon at the right hand side of the login box).

It solved my problems with CCSM, but I'm not sure if yours are the same.

If that works, you should be able to correct the tweaks from there and
go back to 3D if you want to.

I've tried logging in under unity 2d, classic, classic no effects, gnome,
and it's the same every time.

It's possibly a corrupted session file. Have had this myself when
first playing around with Ubuntu :-) The thread here should help..

http://ubuntuforums.org/showthread.php?t=793820

I checked that thread. It was useful.

Logging in using the new user I had created, I used gksudo nautilus to go
into my original user home directory. First I renamed .gnome2 to .gnome2 bak.
I tried to login but the only change seemed to be that the text that had flashed
across the blank screen too fast to read before was gone. Other wise it was the
same. I then went back into the new user and used gksudo nautilus to go into
the original user home directory and renamed .conf to .conf bak. I was
successful. I haven't checked much for any changes but my theme, which I was
changing when this started, was back to default. My Windows title and Menu
fonts are the same as they were after changing them.

Thanks again to everyone for the concern and all the advice. I always tell
my wife that, along with Ubuntu installed on a PC being "a poor man's Mac", the
helpful attitude I USED to see with Mac people is prevalent in the Ubuntu
community. It's one of the main reasons I switched to Ubuntu. I know I can
find some site that discusses my problem, some tutorial telling how me to fix
it, or a group like this with people wanting to help.