Bad packaing of the NVIDIA properiatary drivers causes X11 not to work upon reboot after a Linux kernel upgrade.
Notice the number of users affected by this bug and how long it has remained unfixed.
The problem is reported fixed in the trunk as of 2014-04-23. But I had it occuring in Ubuntu 14.04 LTS Trusty Thar on 2014-04-30. If the big was fixed, it is not in the latest LTS branch.