My Toshiba satellite laptop boots with a blank screen.

I upgraded from Ubuntu 14.04 LTS, to Ubuntu 14.10. When I start my computer, I have a blank screen. If I plugin my laptop to my TV, at first only the TV turns on, but the login screen shows up on my TV, and when I log in my laptop's screen is turned on, and show Unity as expected. However, If I just let Ubuntu boot without any keys pressed, or if I press shift until the GRUB menu shows up and then select the "Ubuntu" option, I get a blank screen. In all situations where I get a blank screen, I hear the startup sound (i.e. I hear drums playing) which indicates that X has started up, just my notebooks screen isn't turned on for some reason. I never had this problem with all versions of Ubuntu, including non-LTS versions, from the earliest one I tried i.e. Ubuntu 12.04 LTS to Ubuntu 14.04 LTS.

Relevant details (that I can think of) are given below:

Type of laptop,
TOSHIBA Satellite L755D-11W: http://www.toshiba.co.il/en/discontinued-products/satellite-l755d-11w/.

`lspci | grep VGA`

    00:01.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI]
    BeaverCreek [Radeon HD 6520G]
    01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Seymour
    [Radeon HD 6400M/7400M Series] (rev ff)

 I cross posted this question here: http://askubuntu.com/questions/543580/i-have-a-blank-screen-after-upgrading-to-ubuntu-14-10.