This started maybe half an hour ago(as of 5:35 pm U.S. Central Time) after I downloaded a security update; I decided to restart the computer soon, and after quickly saving work on some programs, I let the computer do its thing.
However, I had had some really strange issues with Firefox earlier in the day (including losing the ability to restore my previous session, and locking up after only opening a single tab), so, before resuming my work on Ubuntu 14.10 I decided to try to go into Recovery Mode, to see if I could find out what was wrong; fixing broken packages was apparently not an option, so I thought I'd try going into the failsafe graphic mode for a while, thinking that might help.
Unfortunately, I wasn't able to progress beyond the initial warning screen(I tried everything I could think of, using a USB mouse, key combos, etc. nothing worked), so I had to do a hard reboot.
And that, I believe, is where it got to that point of being unable to boot; I even tried loading a previous kernel of Ubuntu 14 which also didn't work-both times, it seemed to load up fine, but then hung up on a black screen, with no indication of a grub error or anything like that.
The good news is, Windows 8 still seems to work okay, so I doubt it's a hard drive related issue, but what could have borked Ubuntu so badly that it suddenly stops booting with no warning? Could the Firefox issues have been a symptom, perhaps?