It happened to me before, and it happened again the day before yesterday: Ubuntu refused to boot, and the only thing that greeted me was a blinking cursor (or dash) in the upper left corner of the blank black screen.
I couldn't remember if there had been an upgrade of the kernel or grub, but put away with that thought that this may be the reason as my first thought was: okay, it happened. Finally the boot drive on that machine died... It had these clicking sounds over some time now, and finally, it must have hit something within the boot area, something essential. Booting into a live CD I looked around the system if the drive showed up. Well, it did and everything was there, not the least amount of erroneous behaviour of the boot drive.
Searching the Internets for an answer, comments pointed me towards grub, which might be misconfigured, or got currupted, and is usually hidden at that early boot stage. That might be the blinking cursor. And from its malfunctioning, the machine would just idle there - with that blinking cursor. So I booted again and tried holding down the left SHIFT key (which usually brings up the grub menu) - but nothing.
That's when I remembered another thread where someone asked another helpless user: "are you sure you are booting from the right drive?". And I remembered that my boot drive for some reason was /dev/sdb while a second drive used for large storage on that machine was /sda... Could it be that grub forgot about which drive was meant for booting and tried booting from the storage drive? I think my config doesn't rely on UUIDs of disks, so it might be possible. After some update... And I think I had this issue some years ago on another machine...
So, try this if you are running a machine with two drives, and after some update you are now suddenly staring at a non-moving non-booting machine, only the blinking cursor greeting you after the BIOS stuff:Open the case, switch SATA cables on your two drives and boot again. For me it worked!