12.04 stuck at the login screen

I had the same problem with Nvidia GITX 580 on 12.04 64bit. Here is the solution:

1. At the lightdm login page, press Ctrl+Alt+F1 to switch to the console (text mode).
2. Log in using your credentials.

3. Run

   sudo apt-get update && sudo apt-get upgrade
   

4. Install nvidia driver:

   sudo apt-get install nvidia-current
   

5. Reboot.
6. Now you should be able to use your keyboard and login in lightdm.

The problem, appears to be lightdm. I uninstalled it, installed gdm. Now it works. Make sure to apt-get update.

I had the same problem. Booting the installation CD worked, to the point where the purple background displays and the bar at the top loads. As soon as the "Install" Welcome screen appeared I would get a blank white box in the middle (where the Try/Install Ubuntu normally is displayed)

I fixed it as follows:

Just as the CD is booting (i.e. when the purple background first appears) press F6. You will get a few options (Try Ubuntu, Install Ubuntu, etc). You will also have the ability to press F6 again to select booting options. Select nomodeset and then proceed to "Install Ubuntu". It will work no problem! (at least it did for me)

System Info:

CPU: Intel i7 2600K
GPU: Nvidia GTX 590
Mem: 16GB DDR3 (1333mhz)
Mobo: Asus P8Z68-V Pro

I had this same problem on my VirtualBox setup. Seems that if the system cannot log in using 3d acceleration, it simply falls back to the login prompt with no other warnings.

I found I was able to log in if I changed to the 2d prompt, installed the VirtualBox drivers and rebooted. So it looks like if you need a 3rd party video driver, you must start in 2d and install the driver first before the system will boot properly.