As I've said in the title, I've stupidly played with my shared libraries without exactly knowing what I was doing. I suddenly couldn't open programs (not even the terminal) so I decided to reboot the computer. On the restart, I faced this black screen of death.
I then tried to boot Ubuntu in recovery mode, but no matter what I tried (either repair broken packages or just to boot into safe mode) nothing made the trick.
Needless to say that I'm a noob with this sort of manipulations and that it should have sufficed me to know that to prevent me from playing around with this stuff.
I've found this answer but I thought it better to ask around before touching anything else. When I follow N0rbert's advice to go on the official Ubuntu packages list, I can't seem to find libz.so.1.
Thank you so much in advance. I've learned my lesson.