Having trouble with your HDMI port not detecting your monitors or TVs all of a sudden? Go to system preferences -- Click Display - Hold down the alt option key -- click the newly revealed Detect Displays button on the bottom right of the Display settings panel. That should solve 90% to 99% of issues if your cables and Macbook pro are good.
This happens to me all the time when I try to plugin to a 4K tv at the office which is the exact same model I have at home. The one at home works while the one at the office never auto detects.
If this did not fix your issue, check that your Macbook Pro HDMI port and HDMI cable ends are clean and free from debri and not damaged, double check that your HDMI cable is working on other devices, make sure your TV or display is powered ON, check that your HDMI cable is securely and fully seated, check that you're on the right input on your TV or Display, make sure the cable is not too long because HDMI has length limits depending on the cable and devices, and then force detect display again on your Macbook pro. If this still did not work, when it worked before. It's likely that your Macbook Pro, TV, or Macbook Pro HDMI port is acting up. I'd recommend taking in your Macbook Pro to the Apple store at that point to get diagnosed. Hope this helped!
