First of all, in Ubuntu 12.04 I had AMD Catalyst Display Driver 12.6 on my HP Pavilion g7-1075dx (ATI Mobility Radeon HD 4250) running great for about one week with my 24" second monitor (as Generic Non-PnP monitor in Windows) 1920x1080 via HDMI cable (through SVGA cable I couldn't get a high resolution). I downloaded the driver from http://support.amd.com/us/gpudownload/linux/legacy/Pages/legacy-radeon_linux.aspx.
I must have not disconnected the 2nd monitor gracefully before shutting down, because when I booted up this morning I got this black screen.
I spent 3 hours getting my laptop to successfully boot up again.
The key is to be able to make changes to the hard drive: Boot up then hold down the Shift key to get the GRUB loader menu. Arrow down to to the first Ubuntu version with '(recovery mode)'. You will get the 'Recovery Menu (filesystem state: read-only)'. You want to change it to write to the file system: arrow down to 'dpkg Repair broken packages' and press enter. Do 'Yes' to remount filesystem in read/write mode. Next, 'apt-get upgrade' will run automatically in the terminal session; let it go and at the end press 'N' to not continue and to not start the upgrade. Then 'apt-get upgrade' will churn some more then say 'Finished, please press ENTER'; press Enter which will then take you back to the Recovery Menu but now you are in filesystem read/write state.
Next, arrow down to 'root Drop to root shell prompt', give the root password, and voila you have a root prompt on a read/write file system.
I tried Apple16's excellent suggestions: (1) I commented out the line in 'blacklist-local.conf' and then once again including commenting out a line in 'blacklist-fglrx.conf' and rebooted but it didn't work.
Also, (2) and (3) did not work for me but they might for you.
Finally, I decided to revert to square zero: Back up '/etc/X11/xorg.conf' and copy 'xorg.conf.failsafe' over it. Then reboot and at least now the PC is usable. Next for me it's back to reinstalling the graphics drivers again.