Ubuntu gets stuck while booting after upgrade

[anekdoteles]

I am relatively new to Linux and had the Ubuntu LTS Jellyfish installed. I never made an upgrade, but found a command that presumably just allows to upgrade to the newest LTS via terminal. After executing and installing however, my machine does not boot up anymore and gets stuck in the Ubuntu loading screen. Where do I start to troubleshoot and what a realistic options?

[currentshaft]

ctr

[anekdoteles]

I think the command that I used could have been this

sudo apt install ubuntu-release-upgrader-core

I also already tried all options after BIOS POST with no success so far.