If it seems you cant use the keyboard and mouse, then, Hold ALT + PrintScreen Keys and type R E. These magic keys will help you to get out of the Frozen Error Box and move into the console. Now Press ALT + F2 to Drop to shell.
Login with your username and password
execute the command, "sudo apt-get update"
execute the command, "sudo apt-get dist-upgrade"
Wait for the operations to complete . Make sure that the downloaded packages are getting installed. If by chance, it fails to download a package, better remove that package and re-run the "dist-upgrade". repeat this till it gets done.
execute "sudo update-grub". Verify that the grub updates its menu with the recent kernel version (2.6.38.x)
Now restart the computer. This should fix the issue
In case you get the same error after re-boot, drop to shell by following the sequence said above. remove the following packages, unity and restart.