After the initial installation of Ubuntu, Ubuntu will boot up but only show the desktop background. Any idea why?

After the initial installation of Ubuntu, Ubuntu will boot up but only
show the desktop background. Any idea why?

After the initial installation of Ubuntu (where everything works fine),
Ubuntu will boot up but only show the desktop background (doesn't ask me
to log in). Any idea why?
I can still press ctrl+alt+f1 to get to terminal if needed. (I also have
OpenSSH installed) My current theory is it is a graphics card problem
(because that always seems to be the problem). If it matters, this happens
with both a full install of Ubuntu and with a Wubi install, but it does
not happen with a virtualbox install.
Currently my system is made up of: AMD 8350, GTX 660, 24 GB of ram, 1 TB
HDD, 128 GB SSD, 500 GB HDD. I have tried installing Ubuntu on all 3 of
them, and it does not work on any of them.