If you are those of almost everyone that has [testing] enabled on /etc/pacman.conf most sure is lately your X server went broken on your last upgrade.
If you get to see the log, you’ll see that message:
This server has a video driver ABI version of 11.0 that this driver does not officially support. Please check http://www.nvidia.com for driver updates or downgrade to an X server with a supported driver ABI.
(EE) NVIDIA: use the -ignoreABI option to override this check.
(EE) Failed to load module 'nvidia' (module requirement mismatch, 0)
(EE) No drivers available
If you see that, then you’re lucky, because I have the solution to your headache.
It is a issue on latest Xorg version (1.11) on testing. To resolve the annoying problem, you must edit /etc/pacman.conf and comment [testing] repositories and then downgrade xorg with
# pacman -Suu
You can leave commented [testing] or if you want desperately a package on testing for some obscure reason, you can always uncomment the directive and the repo and add on the directive IgnorePkg on /etc/pacman.conf the following
IgnorePkg = xorg xorg-server-common xorg-xinit xorg-server xf86-input-evdev
But I recommend to inmediatly comment back IgnorePkg directive and [testing] repository. Thus, when Xorg 1.11 will be stable, automatically will be upgraded.