Can't Boot: "Failed to isolate default target, freezing"

After executing "apt-get upgrade" in Terminal, the command was doing what it was supposed to do, but after a while, I come back to see it's progress and it was frozen. I shut it down, using the Virtual Machine Tools, then when I try to start it, it starts normally to this Screen:

https://i.gyazo.com/7392557791fde88f...10aa971be7.png

However, every time I select the first option, this pops up:

https://embed.gyazo.com/41fa2ca365a5...bc14a6f46f.png

It really bothers me, as I've got loads of files into my System. Is there a way to fix this error? I haven't been able to find any solutions out there on the internet.

Thank you in advance