Ubuntu Xenial 16.04 users who updated to receive the Meltdown and Spectre patches are reporting they are unable to boot their systems and have been forced to roll back to an earlier Linux kernel image. The issues were reported by a large number of users on the Ubuntu forums, Ubuntu’s Launchpad bug tracker, and Reddit thread. Only Ubuntu users running the Xenial series appear to be affected. Canonical deployed kernel image 4.4.0-108 as part of a security update for users yesterday, on January 9.
Source: https://www.bleepingcomputer.com/news/software/meltdown-and-spectre-patches-causing-boot-issues-for-ubuntu-16-04-computers/