Haha, i just wanted to make a PSA Thread about that.
I had the same problem today. After upgrading with pacman -Syu, nvidia-dkms and other packages upgraded to lastest version. And what already maked me curious that i didnt saw in the upgrade process that he rebuilded the dkms modules to the new version.
First i thought i simply missed the output and restarted. And voila. No X and so no Desktop.
Quick log reading showed me that he was still trying to load the old builded kernel modules and crashed with the Output:
NVRM: API mismatch: the client has the version 495.46, but
NVRM: this kernel module has the version 495.44. Please
NVRM: make sure that this kernel module and all NVIDIA driver
NVRM: components have the same version.
The simple fix was to rebuild the dkms modules with simply force reinstall nvidia-dkms.