I experienced this issue (or something similar) yesterday. I used the alt-sysrq-1 and alt-sysrq-t combo mentioned in the document linked to in the last comment and after sometime the machine started responding again. I captured the kern.log and will attach it here in case it's useful.
** Attachment added: "kern.log" https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1583246/+attachment/4697121/+files/kern.log -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to xorg in Ubuntu. https://bugs.launchpad.net/bugs/1583246 Title: System freezes after returning from locked screen Status in xorg package in Ubuntu: Incomplete Bug description: Quite frequently when I lock my screen (or it locks itself) and I unlock it the system is responsive for a few seconds but then completely freezes. I can't reboot but have to turn off the power on the machine. ProblemType: Bug DistroRelease: Ubuntu 16.04 Package: xorg 1:7.7+13ubuntu3 ProcVersionSignature: Ubuntu 4.4.0-22.40-generic 4.4.8 Uname: Linux 4.4.0-22-generic x86_64 NonfreeKernelModules: nvidia_uvm nvidia_modeset wl nvidia .proc.driver.nvidia.gpus.0000.02.00.0: Error: [Errno 21] Is a directory: '/proc/driver/nvidia/gpus/0000:02:00.0' .proc.driver.nvidia.registry: Binary: "" .proc.driver.nvidia.version: NVRM version: NVIDIA UNIX x86_64 Kernel Module 361.42 Tue Mar 22 18:10:58 PDT 2016 GCC version: gcc version 5.3.1 20160413 (Ubuntu 5.3.1-14ubuntu2) .tmp.unity_support_test.0: ApportVersion: 2.20.1-0ubuntu2 Architecture: amd64 CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins' CompositorRunning: compiz CompositorUnredirectDriverBlacklist: '(nouveau|Intel).*Mesa 8.0' CompositorUnredirectFSW: true CurrentDesktop: Unity Date: Wed May 18 11:24:59 2016 DistUpgraded: Fresh install DistroCodename: xenial DistroVariant: ubuntu DkmsStatus: bbswitch, 0.8, 4.4.0-22-generic, x86_64: installed bcmwl, 6.30.223.248+bdcom, 4.4.0-21-generic, x86_64: installed bcmwl, 6.30.223.248+bdcom, 4.4.0-22-generic, x86_64: installed nvidia-361, 361.42, 4.4.0-22-generic, x86_64: installed EcryptfsInUse: Yes ExtraDebuggingInterest: Yes GraphicsCard: Intel Corporation 4th Gen Core Processor Integrated Graphics Controller [8086:0416] (rev 06) (prog-if 00 [VGA controller]) Subsystem: Dell 4th Gen Core Processor Integrated Graphics Controller [1028:060d] Subsystem: Dell GK107GLM [Quadro K1100M] [1028:060d] InstallationDate: Installed on 2016-04-10 (37 days ago) InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Beta amd64 (20160323) MachineType: Dell Inc. Dell Precision M3800 ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.4.0-22-generic root=/dev/mapper/ubuntu--vg-root ro quiet splash SourcePackage: xorg Symptom: display UpgradeStatus: No upgrade log present (probably fresh install) dmi.bios.date: 08/17/2015 dmi.bios.vendor: Dell Inc. dmi.bios.version: A10 dmi.board.name: Dell Precision M3800 dmi.board.vendor: Dell Inc. dmi.board.version: A10 dmi.chassis.type: 8 dmi.chassis.vendor: Dell Inc. dmi.chassis.version: Not Specified dmi.modalias: dmi:bvnDellInc.:bvrA10:bd08/17/2015:svnDellInc.:pnDellPrecisionM3800:pvrA10:rvnDellInc.:rnDellPrecisionM3800:rvrA10:cvnDellInc.:ct8:cvrNotSpecified: dmi.product.name: Dell Precision M3800 dmi.product.version: A10 dmi.sys.vendor: Dell Inc. version.compiz: compiz 1:0.9.12.2+16.04.20160415-0ubuntu1 version.ia32-libs: ia32-libs N/A version.libdrm2: libdrm2 2.4.67-1 version.libgl1-mesa-dri: libgl1-mesa-dri 11.2.0-1ubuntu2 version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A version.libgl1-mesa-glx: libgl1-mesa-glx 11.2.0-1ubuntu2 version.nvidia-graphics-drivers: nvidia-graphics-drivers-* N/A version.xserver-xorg-core: xserver-xorg-core 2:1.18.3-1ubuntu2 version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.10.1-1ubuntu2 version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:7.7.0-1 version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20160325-1ubuntu1 version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.12-1build2 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1583246/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp
