I have a Dell Inspiron 15 3000, it came with Ubuntu 12.04, I wanted to upgrade it to 14.04, but when I try to boot from USB, after I choose start from the menu (where there is install, check memory etc) all I get is a blinking cursor on black background. This is the model with AMD hardware.
Run the system distribution upgrade from within Ubuntu 12.04. Just be sure to set it to only recognize the LTS releases. There's no reason to do a clean install from USB.