When I try to login with lightdm my monitors go dark and then switch off before taking me back to the lightdm login screen. I get a message telling me:

Could not switch to the monitor configuration

could not set the configuration to CRTC 64

I'm running unity in a multi-monitor set up with 3 monitors. Before this error started occurring I had 2 monitors active and one switched off. I played Minecraft and upon closing the program there seems to have been some kind of problem setting the resolution and all of my screens went black except a small (about 640x480) window in the top left hand corner of the first monitor.

I tried using the display manager to fix the resolution but after applying the settings the screens both displayed in some kind of 4:3 resolution (1280x1024 maybe). I figured maybe I needed to log out and in again, and ever since then I haven't been able to start a unity session.

I installed xfce4 and I can log into that but really I want unity back. I have tried reinstalling ubuntu-desktop and unity, but I'm still receiving the same error. I've tried checking the permissions for .Xauthority and .ICEauthority, but they look fine.

Here is my output for lshw -c video:

   description: 3D controller
   product: GF117M [GeForce 610M/710M/820M / GT 620M/625M/630M/720M]
   vendor: NVIDIA Corporation
   physical id: 0
   bus info: pci@0000:01:00.0
   version: a1
   width: 64 bits
   clock: 33MHz
   capabilities: pm msi pciexpress bus_master cap_list rom
   configuration: driver=nvidia latency=0
   resources: irq:47 memory:f6000000-f6ffffff memory:e0000000-efffffff memory:f0000000-f1ffffff ioport:e000(size=128) memory:f7000000-f707ffff
   description: VGA compatible controller
   product: 3rd Gen Core processor Graphics Controller
   vendor: Intel Corporation
   physical id: 2
   bus info: pci@0000:00:02.0
   version: 09
   width: 64 bits
   clock: 33MHz
   capabilities: msi pm vga_controller bus_master cap_list rom
   configuration: driver=i915 latency=0
   resources: irq:44 memory:f7400000-f77fffff memory:d0000000-dfffffff ioport:f000(size=64)

Everything looks good if I modinfo i915, but if I modinfo nvidia I get modinfo: ERROR: Module nvidia not found. nvidia should be my default display driver.

Here is the contents of my xorg.conf:

Section "ServerLayout"
Identifier "layout"
Screen 0 "nvidia"
Inactive "intel"

Section "Device"
Identifier "intel"
Driver "intel"
BusID "PCI:0@0:2:0"
Option "AccelMethod" "SNA"

Section "Screen"
Identifier "intel"
Device "intel"

Section "Device"
Identifier "nvidia"
Driver "nvidia"
BusID "PCI:1@0:0:0"
Option "ConstrainCursor" "off"

Section "Screen"
Identifier "nvidia"
Device "nvidia"
Option "AllowEmptyInitialConfiguration" "on"
Option "IgnoreDisplayDevices" "CRT"

What further steps should I take to further diagnose this problem and restore my unity session?

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