This is likely related to the NVIDIA graphics card. There are lots of posts discussing this problem with lots of different proposed solutions, but what worked for me was updating the drivers. I originally tried updating the drivers by downloading them from the NVIDIA website, but the solution was this line of code to update the drivers. You should check the NVIDIA website just to get the driver number (might vary per device). Also, prior to this fix, the HDMI was not found on my computer, but I could use the USB-C and an adapter to connect to another monitor.
sudo apt install nvidia-driver-435
Ubuntu 18.04.3 cant find second monitor - Dell G3 - NVIDIA 1060 TI
Here's a list of other fixes people have proposed.
External monitor not detected on Ubuntu 18.04