Comment 2 for bug 768378

"Everything is looking without theme" sounds like a window manager crash. I *think* all the symptoms you describe correspond to a WM crash, although blank screens with moving mouse issues can also sometimes be GPU lockups.

For GPU lockup bugs with Intel graphics, you need to collect the output of 'dmesg' and your /sys/kernel/debug/dri/0/i915_error_state file. Both of these must be collected while the machine is locked up (e.g. by sshing into the sick machine over ethernet). See https://wiki.ubuntu.com/X/Troubleshooting/Freeze for additional info.