I upgraded to 12.04 today from 11.10 from a bootable thumbdrive. The upgrade took approximately 8 hours. I had to make a new user, and I'm unable to sign into my user that I used in 11.10. I can see my files in the /home/ directory, but there isn't an option to sign into that user on the boot screen. I am also unable to use the normal "Ubuntu" setting, I get a black screen with that. I have to use "Ubuntu 2D". Please help.
You have a permissions problem.
What you need to do is get to a terminal as explained here, and sign in with your username and password.
Then, you need to run the following:
Once that's done, you should be able to log in via lightdm.
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A workaround could be : move you old user folders and files to your new user home.
If you want to keep all your setting you must view the hidden folders from nautilus (view > show hidden files) then copy the hidden folders of your choice AKA application you will still use.
Copy/Replace the entire usr/previous user/home with your new /usr/current user/home
Go to user a/c and delete the old one with all files