I recently broke my dual boot installation (Windows & and Ubuntu) during an upgrade -- supposedly to fix missing dependencies but unwittingly upgraded from trusty to vivid, and now vivid won't boot up to the UI, obviously due to driver issues and now I've got a black screen.
In fact, initially grub went crazy and I couldn't boot up to Windows as well until I booted up from the Ubuntu bootable USB drive the grub issue "resolved itself". But the Ubuntu installation is still booting to blackscreen. I can select the recovery mode (recovery/dpkg) option but I get the error message upgrade from vivid to trusty is not supported. I can also bootup Ubuntu to terminal with networking support. I've tried loads up options to bootup without the nVidia driver but NONE of them work.
How can I fix my installation without having to reinstall Ubuntu in a way that it messes up working applications in the previous install? I don't mind staying on vivid or rolling back to trusty as long as my installation works.
Update
Here's what my screen looks like when I boot up
