Can you load a recovery kernel from Advanced Options for Ubuntu? If so, then select Network to get a connection to the internet. Then select Root to get to a root shell (terminal). We exit the command line and get back to the recovery menu by typing exit.

At the root shell we can remove the Nvidia driver with

apt remove --purge nvidia*
When back at the recovery menu Resume will load the Ubuntu desktop with an open source video driver. If you get a black screen, then re-boot. If you cannot load a recovery kernel you may have work from a Live session to install a 5.4 kernel.