I am sure you are not looking forward to trouble shooting more than you already have, but .. Do you have another monitor to try? Even a TV that accepts HDMI input? The thing is to make sure that either nothing displays using the HDMI port, or that some devices do and others do not. Do you have another cable you can try?
Another thing, not very likely, but still possible. If there are more than two places to connect an HDMI cable, could it be you are on the wrong one?