This is a problem common with Nvidia cards. If you do have an Nvidia video card, you are going to need to load the driver.
First of all do a
uname -a, it will show something like
4.15.0-142-generic, not exactly that, but something like that in the output.
After that do
sudo apt install linux-headers-4.15.0-142 linux-headers-4.15.0-142-generic.
It's not going to be that exact version, but the numbers have to match whatever
uname -a gave you.
Then you are going to add the graphics driver repository:
sudo add-apt-repository ppa:graphics-drivers/ppa
Now you can check what driver is available:
sudo ubuntu-drivers devices
It should display a list and some entry is going to say "recommended" next to it. So you can go ahead and install it.
sudo ubuntu-drivers autoinstall
Make sure to reboot and you should be all set. All that said, I really do hope that somebody fixes the issue that linux-headers packages are never part of any dependencies even though you can't load the video drivers correctly without them. 16.04 had a huge problem where it lost its mind every time the kernel updated. Every time there was a kernel update, before rebooting had to remember to go in and manually install the correct header set. Otherwise pain was inevitable.