Two weeks ago I updated my linux and then the next time I connected to a HDMI is was not working. After some hours I realized that the problem is… like always… the drivers of the graphic card. I am going to write here how I fixed it:

• Open your Ubuntu with Ubuntu GUI (you can use other but it easier with this), and open the application Additional Drivers, and select the driver from NVIDIA, the one that is working for me now is the 470 server, latter I used the server version on the latest version. Push Apply. The one I had selected and it was not working was the open driver from nuveau, that was giving errors.
• Then it will show something about secure key, put there a password
• THIS IS THE IMPORTANT PART when you reboot it will show a message, with the options boot and enroll key. Press ENROLL KEY and put the password you put in the last step. With that everythin will be perfect.