I experienced this issue (or something similar) yesterday. I used the alt-sysrq-1 and alt-sysrq-t combo mentioned in the document linked to in the last comment and after sometime the machine started responding again. I captured the kern.log and will attach it here in case it's useful.
** Attachment added: "kern.log" https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1583246/+attachment/4697121/+files/kern.log -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1583246 Title: System freezes after returning from locked screen To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1583246/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
