I logged into my Gentoo today only to find the X session freeze as soon as I launched it. The logs revealed the following output repeated multiple times:
(EE) module ABI major version (12) doesn't match the server's version (13) (EE) Failed to load module "evdev" (module requirement mismatch, 0) (EE) No input driver matching `evdev'
As I had updated over 80 ebuilds the previous time I ran the system I had a long list of possible culprits. Additionally I noticed my / partition was 100% full, so anything could have gone awry the last time I ran the machine.
After first emerging all of the updated x11 related packages again nothing changed: X session would still freeze. What I should have done in the first place is reinstall all of the x11-related packages, not just the ones I had updated. So the solution is a simple
$ emerge $(qlist -IC x11-drivers)