After upgrading to Ubuntu 17.10 my secondary screen cannot be detected anymore. I have an AMD Radeon HD 7700M card and my monitor is connected through HDMI or DisplayPort, neither connection works. In Ubuntu 17.04 I had the possibility to revert to an older kernel and then it worked, but now even that older kernel is not working anymore.
I have a Nvidia 340 driver for my G92 GeForce GTS250 graphics card. I had the monitor not detected the problem after a kernel upgrades from Kernel 4.10.0-42. I just tried out the open source driver and I don't know why but it works and I can get the full-screen showing. Before I had a big black band around the display. I don't know what problems I will have with the open source driver but if nothing serious goes I have to use it or go back to an older kernel. I am using Linux Mint 18.3. This is only a a possible work around if the original driver is proprietory. The open source doesn't take advantage of many of the features of the graphics card I understand. For my purposes I have a functioning monitor.