Comment 6 for bug 1760377

Had these exact symptoms (lockup, not even sysrq working) on Ubuntu 16.04 with kernel 4.4.0-116 and 4.13.0-37 (booting to the "prior stable" kernel in each case, 4.4.0-112 and 4.13.0-36, worked successfully). There's quite a lot in the changelog (http://changelogs.ubuntu.com/changelogs/pool/main/l/linux-hwe/linux-hwe_4.13.0-37.42~16.04.1/changelog) so I wouldn't like to speculate on what exactly caused this, but I did find a fix in my case: remove the intel-microcode package.

I have a vague recollection of installing this from the "Additional Drivers" section of the updates system, but after booting to the older kernel, and running "sudo apt purge intel-microcode" I found I could successfully reboot in the newer kernel.