Ubuntu – Enable Wi-Fi in Recovery Terminal

After booting to recovery mode select the option 'remount' as suggested in the comment above.
Drop into a root shell with internet acces ('netroot' option) and try executing the following commands to reset your graphics driver configuration to the ubuntu default drivers.

sudo apt-get remove --purge xorg-driver-fglrx fglrx*
sudo apt-get remove --purge fglrx*
sudo apt-get install build-essential cdbs fakeroot dh-make debhelper debconf libstdc++6 dkms libqtgui4 wget execstack libelfg0 dh-modaliases
sudo apt-get install --reinstall libgl1-mesa-glx libgl1-mesa-dri
sudo mv /etc/X11/xorg.conf /etc/X11/xorg.conf.old
sudo dpkg-reconfigure xserver-xorg

Restart your computer using

sudo reboot

And boot up normally

Or if you have installed the drivers manually from the binary download available on the amd website run these commands:

 sudo sh /usr/share/ati/fglrx-uninstall.sh
 sudo mv /etc/X11/xorg.conf /etc/X11/xorg.conf.old
 sudo apt-get install --reinstall xserver-xorg-core libgl1-mesa-glx libgl1-mesa-dri
 sudo dpkg-reconfigure xserver-xorg
 sudo reboot

First of all what is the model of you graphic card ? It can be found using this command:

    lspci | grep VGA

I faced the same kind of issue recently. I had a Ubuntu 13.10 with a Radeon HD 3850 using fglrx-legacy drivers, and I was forced to remove them in order to upgrade to Ubuntu 14.04. The 14.04 generic display driver is ok but only provides a software rendering mode. I confirm that the standard fglrx drivers won't work (black screen) with old ATI cards such as the HD2xxx-4xxx series.

Once the 14.04 installed you can try to install fglrx-legacy drivers with those commands (worked for me on 13.04+, not tested yet on 14.04) or via Synaptic:

    sudo add-apt-repository ppa:makson96/fglrx
    sudo apt-get update
    sudo apt-get upgrade
    sudo apt-get install fglrx-legacy

more info in this post : Radeon HD 2000, 3000, 4000 on 12.10 Quantal: fglrx (legacy) 12.6 unsupported, what to do?

You can test if the drivers are properly working by using the following command

    glxinfo | grep render

Video drivers are always the painful part. I am also interested. Please keep us informed if you manage to solve your problem !