I know this is from a long time ago - but I just had this same thing happen to me so I figured I'd let you know what the issue was for me. When I added a graphics card my machine would boot, but I couldn't ssh into it, but the normal screen would show up. The issue was my ethernet interface name would change when the graphics card was plugged in. Normally it was a enp5..., but when I put in the new graphics card, it became enp6..., so the interface reference in my netplan file wouldn't work. I just had to change my netplan yaml file from enp5 to enp6, apply, and it worked fine.