I just followed these instructions for a problem that my wife was having with a X1 Carbon (4th gen - 20FB) after it took on Lenovo updates a few days ago. Essentially after updating the graphics driver, the screen went blank and the only way to interact with the laptop was thru a HDMI connected screen. I was able to get the laptop screen back when I switched to the generic Microsoft graphics driver but then you don't have use of the HDMI port. The solution in the instructions has restored the correct screen functionality including the use of the HDMI port.
On observation, the fix as proposed above had me install an intel 520 graphic driver dated 2015-12-15 version 20.19.15.4352
This hardly seems like a fix or a patch... this appears more to be avoiding a newly introduced problem... I also happen to have an X1 Carbon (4th gen - 20FC) machine which is essentially the same specs (cpu, ram, screen etc.) mostly just a different purchase date. My machine took on Lenovo Updates on the same day my wife's machine did and my machine is happily displaying it's screen using an intel 520 graphics driver dated 2017-01-18 version 21.20.16.4590 (the same driver which is the kiss of death to my wife's screen).
I have next day onsite warranty with the laptop, have placed a service call and I am now holding off the technician until I had an opportunity to try this fix... He is ready to swap out the motherboard on the laptop... Does anyone know if this is a 'bad batch' of Intel 520 chipsets? I've come across a number of very recent, similar complaints related to X1 and X1 Yoga.
If anyone has comments I'd be interested in hearing them, otherwise I'm tempted to go for the motherboard swap even though I hate to open a machine up unless its really necessary.
Thanks - Tim