Ubuntu 12.04 froze, requiring powercycle. What should I look / grep for in the logs?

Solution 1:

A freeze sounds a lot like an error either in Xorg or in the kernel. Have you tried to ssh to the box? Sometimes that works even if local user interfaces are all irresponsive. A working ssh connection might be very helpful debugging this kind of issue.

From the data you provided, I'd go for the kernel log. It seems libreoffice crashed for some reason, and within a second, chrome was gone as well. So there was definitely some kind of problem at that time. Have a closer look at the log around those error lines, and see whether you can identify the first error message of that event cascade. The timestamps to the left should be useful, as any event related to the freeze should be pretty close to the two I mentioned.

Just judging from the two events included in your log, it seems likely that the libreoffice crash somehow made X unstable, causing both the UI freeze and the chrome crash. You will notice that chrome crashed in an X library.