First of all, make sure that your HDMI cable is working perfectly as expected. If all ok, then apply the below given tricks:
Solution 1: Go with the Display Output Settings to Fix HDMI Monitor Not Working on Windows 10
Solution 2: Update Graphics Driver to Repair Windows Does Not Recognize Second Monitor
Solution 3: Run Hardware and Device Troubleshooter to Fix Monitor Not Detected by PC
Solution 4: Verify that your HDMI device is the Default Device
Solution 5: Run System Restore