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-X, which is subscribed to xorg in 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 _______________________________________________ Mailing list: https://launchpad.net/~ubuntu-x-swat Post to : ubuntu-x-swat@lists.launchpad.net Unsubscribe : https://launchpad.net/~ubuntu-x-swat More help : https://help.launchpad.net/ListHelp
