Ubuntu 20.04 boots to black screen which says /dev/nv*******: clean, 508***/650**** files, 247*****/260******* blocks
To my great surprise, df -h showed me that my disk usage on my Ubuntu partition was 100%. I had assumed that almost all of my disk usage would be from my home directory. But it turns out that var and snap are quite large as well.
Solution: I booted into recovery mode from the GRUB menu and deleted some files. Then my system was able to boot up as normal.
This thread was particularly helpful in troubleshooting. "dev/sda1: clean, ..." This message appears after I startup my laptop, then it won't continue booting