Performing a reset is great as we can dismiss any Connect App issues.
Is it only the HDMI port?
Did it ever work before?
You can do the following tests:
- Try a different HDMI cable (a short cable, bypass any HDMI switches, desk ports and so on)
- Try with a different laptop
- When connected to the HDMI port, toggle the Video input button (under the power button, side panel) until you see HDMI showing on the bottom right corner of the screen.
If there is still no image after this, you should open a case with Support as the issue might be hardware. You can check for damage to the port, especially if the Surface Hub is on a rolling stand, as sometimes users roll the device away with cables attached.