This happened when I tried to upgrade from 12.04 to 12.10. I don't know whether the upgrade process had completed downloading and installing packages or whether it was still in progress, but I got a message
The application Compiz has closed unexpectedly.
At that point, my mouse stopped working so I switched to
tty1 by Ctrl+Alt+F1 and rebooted from there.
I then found that I could not boot into Ubuntu any more. My screen stayed first purple then black. I went back to
tty1 and it said I was running 12.10.
I tried some suggestions in this thread on Ubuntuforums: Wubi blank purple screen than blank black screen and this one: How to set NOMODESET and other kernel boot options in grub2 but nothing seemed to work.
I can boot Windows as normal. I am using Wubi on Windows7. My laptop is a Lenovo T410i with NVIDA NVS 3100m