I noticed this after returning to my desk, so the display had gone into powersave mode. Attempting to unlock seemed to succeed, but the session was unusable. The nouveau driver was logging interesting things.
ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: unity 7.2.0+14.04.20140402-0ubuntu1
ProcVersionSignature: Ubuntu 3.13.0-21.43-generic 3.13.8
Uname: Linux 3.13.0-21-generic x86_64
ApportVersion: 2.14-0ubuntu1
Architecture: amd64
CompizPlugins: [core,composite,opengl,decor,mousepoll,grid,vpswitch,compiztoolbox,imgpng,gnomecompat,regex,move,place,resize,snap,unitymtgrabhandles,wall,animation,session,expo,workarounds,fade,ezoom,scale,unityshell]
Date: Wed Apr 2 16:51:49 2014
DistUpgraded: 2014-01-22 19:30:50,196 DEBUG enabling apt cron job
DistroCodename: trusty
DistroVariant: ubuntu
EcryptfsInUse: Yes
GraphicsCard:
NVIDIA Corporation G73 [GeForce 7600 GT] [10de:0391] (rev a1) (prog-if 00 [VGA controller])
Subsystem: BFG Tech Device [19f1:201f]
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.13.0-21-generic root=UUID=cd444f9d-672e-4d7b-aea8-657fff0ea1f4 ro quiet splash crashkernel=384M-:128M crashkernel=384M-:128M crashkernel=384M-:128M nomdmonddf nomdmonisw crashkernel=384M-:128M nomdmonddf nomdmonisw crashkernel=384M-:128M nomdmonddf nomdmonisw crashkernel=384M-:128M vt.handoff=7
SourcePackage: unity
UpgradeStatus: Upgraded to trusty on 2014-01-23 (69 days ago)
dmi.bios.date: 06/26/2012
dmi.bios.vendor: Intel Corp.
dmi.bios.version: WBIBX10J.86A.0336.2012.0626.0141
dmi.board.asset.tag: Base Board Asset Tag
dmi.board.name: DP55WB
dmi.board.vendor: Intel Corporation
dmi.board.version: AAE64798-207
dmi.chassis.type: 2
dmi.modalias: dmi:bvnIntelCorp.:bvrWBIBX10J.86A.0336.2012.0626.0141:bd06/26/2012:svn:pn:pvr:rvnIntelCorporation:rnDP55WB:rvrAAE64798-207:cvn:ct2:cvr:
version.compiz: compiz 1:0.9.11+14.04.20140328-0ubuntu1
version.ia32-libs: ia32-libs N/A
version.libdrm2: libdrm2 2.4.52-1
version.libgl1-mesa-dri: libgl1-mesa-dri 10.1.0-4ubuntu1
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 10.1.0-4ubuntu1
version.xserver-xorg-core: xserver-xorg-core 2:1.15.0-1ubuntu7
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.8.2-1ubuntu2
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:7.3.0-1ubuntu3
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.910-0ubuntu1
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.10-1ubuntu2
xserver.bootTime: Wed Apr 2 16:51:01 2014
xserver.configfile: default
xserver.devices:
input Power Button KEYBOARD, id 6
input Sleep Button KEYBOARD, id 7
input Logitech Optical USB Mouse MOUSE, id 8
input BTC USB Multimedia Keyboard KEYBOARD, id 9
input BTC USB Multimedia Keyboard KEYBOARD, id 10
xserver.errors:
Failed to load module "nvidia" (module does not exist, 0)
Failed to load module "nvidia" (module does not exist, 0)
NOUVEAU(0): failed to set mode: Permission denied
NOUVEAU(0): failed to set mode: Permission denied
xserver.logfile: /var/log/Xorg.0.log
xserver.version: 2:1.15.0-1ubuntu7
xserver.video_driver: nouveau
In case it's relevant: I filed this from the command line (typing the launchpad.net URL into a browser on another PC by hand). After rebooting the system described in this report, apport prompted me to file bug 1301615.