On my Lenovo W510 with an NVidia GT216GLM [Quadro FX 880M], starting the X-server with the NVidia 340xx driver newer than 340.46 FUBARs the graphic subsystem completely, i.e., there is a black screen, the backlight is off (and no image showing anyway), there is no possibility to switch back to a terminal etc. Magic SysRq works though… This occurs both with and without an external monitor on DisplayPort or on a DisplayPort->HDMI cable.
A NULL pointer dereference occurs somewhere in the NVidia driver. You can find a stack trace in this ArchLinux forum post I wrote some time ago: https://bbs.archlinux.org/viewtopic.php?pid=1474892#p1474892 This occurs when the X-server starts, way before any GUI elements appear. This is most likely the reason why my system ends up in an undefined state with backlight off and unable to display anything.
The problem persists with kernels 3.16.x, 3.17.x as well as 3.18.x. The 340.46 driver works perfectly though (with all those kernels). My distribution is ArchLinux, but I’m using a custom kernel. I can provide my kernel .config if it’s relevant (but as a matter of fact, the NVidia drivers have always worked fine with all of my custom kernels … up to 340.46).
If you need more details, please just let me know how to gather them for you.