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 :

*-display 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 *-display 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" EndSection Section "Device" Identifier "intel" Driver "intel" BusID "PCI:0@0:2:0" Option "AccelMethod" "SNA" EndSection Section "Screen" Identifier "intel" Device "intel" EndSection Section "Device" Identifier "nvidia" Driver "nvidia" BusID "PCI:1@0:0:0" Option "ConstrainCursor" "off" EndSection Section "Screen" Identifier "nvidia" Device "nvidia" Option "AllowEmptyInitialConfiguration" "on" Option "IgnoreDisplayDevices" "CRT" EndSection

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