After KernelUpgrade Loadable Modules
Have you ever wondered, why your USB stick isn't recognized on your shiny laptop?! Well, it may be the case that you ran pacman -Syu and it included a linux kernel update.
git clone https://github.com/bebehei/akulm.git
cd akulm
makepkg -i
It'll probably be available in AUR shortly.
akulm provides some pacman hooks. These hooks instruct pacman to copy the modules of the currently used kernel into /run/modules and provide a link in /usr/lib/modules. So upon reboot, the only debris left is a symlink.
In comparison to other distros, ArchLinux only has got one kernel available at all times. While for example on debian, many linux-image-* packages are installed simultaneously. So pacman has to remove the module files, which are actually still required by the current running kernel. Any call to insmod/modprobe fails.
You can't downgrade to your current kernel version after you previous upgrade. You have to remove the symlink manually before invoking a downgrade. An akulm clean should suffice.