If your laptop has an Intel 8260 wireless adapter and you have upgraded to the 4.12 kernel, your wifi probably isn’t working anymore. This is because the minimum firmware API version has been updated to 22 in a recent commit to the Intel 8000 series driver source code (in iwl-8000.c):

To fix this problem you need to roll back to your previous kernel (assuming wifi was working before you installed the 4.12 kernel), and then install the latest firmware for the Intel 8260, which is (at time of writing) version 27.

Remove Kernel 4.12

This step isn’t required if you have a wired connection that is working, or you’re able to obtain the latest firmware on another computer (and copy it to the laptop you’re trying to fix).

sudo apt remove linux-headers-4.12*
sudo apt remove linux-image-4.12*
sudo reboot

Install Latest 8260 Firmware

Linux firmware can be found here.

To install it, start a terminal and run the following:

cd /lib/firmware
sudo wget https://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git/plain/iwlwifi-8000C-27.ucode

This will install version 27 of the Intel 8000 series wireless adapter firmware. Reboot, and you’ll be good to go.