I have the same problem on my Lenovo y580 Laptop. It has an Intel HD Graphics 4000, and about six months ago, the HDMI output to an HDTV stopped working.
I saw this post hoping for some solution, but it still isn't working without problems.
If I roll back to the original driver, I can get a picture to go through with the HDMI cable, but then when I restart my computer it becomes unresponsive again. Then I update it to the latest driver and it will work again until I reboot the computer.
I think it has something to do with the driver itself. The registry of the computer when it is working compared to when it isn't working has a large difference in file size. There has to be a better solution rather than rolling back/updating the driver every time I want to use my computer with an HDTV.
Please help, I have found nothing resolving this problem yet, and it seems like I am not the only person.