I tried to upgrade my Ubuntu 14.04 using the Software Updater. The update seemed to be progressing normally except it skipped the upgrade to 14.10 and was upgrading to 15.10 instead of 14.10. When I returned from lunch there was a black screen with a small, blinking white underline character in the top left corner, and I couldn't start the GUI desktop environment.
My system has 2 hard disks: a 458GB Sandisk SSD and a 120GB Crucial SSD. If I boot to the Sandisk SSD I get the result described above. If I boot to the Crucial SSD I get a brief purple screen as in Ubuntu, then just a black screen with a very small stationary white underline character in the top left corner. If I now press Ctrl+Alt+Delete I get a brief flash of a purple screen with the word ubuntu in large white letters, then the black screen with the white underline character returns, but I still can't get to a GUI desktop environment.