I have recently experienced problems with Ubuntu 20.04 after an update included updates to the NVIDIA graphics drivers. A normal boot hung on a black screen showing only
'dev/sda1: clean, 552599/6111232 files, 7119295/24414464 blocks'
It took me a while to work out how to fix this as it was not immediately apparent that the issue was caused by NVIDIA. In the end, what fixed it for me was to boot into recovery mode (on startup choose “ubuntu advanced options, ubuntu recovery mode”, opening “Software and Upgrades” and reverting the installed graphics driver from the proprietary NVIDIA driver to the open source xorg driver. On a reboot, the OS started up OK. However, I quickly came to notice some buggy behaviour, such as a failure to share the display across two screens. Some web searching suggested this was a known problem with the open source driver so I decided to purge the nvidia drivers on my system and reinstall the recommended driver for my GPU (GeForce GTX 1650) which happened to be the nvidia-driver-440.
After a reboot, this does seem to have fixed the issue. I’ll update this posts if I uncover any issues.