Over the past week many Linux distributions have published software updates to deal with the CPU flaws commonly referred to as Meltdown and Spectre. Unfortunately, some kernels which include the fix are causing problems of their own. For instance, some Ubuntu 16.04 users who installed kernel version 4.4.0-108 found their systems would not boot with the new kernel. Rebooting the machine and selecting an older kernel from the boot menu works around the issue. Canonical quickly published a new kernel update, version 4.4.0-109, which fixed the issue. More details can be found in this Launchpad issue report.