After upgrading my Ubuntu 14.04 LTS install from linux kernel 3.13 to 3.16, USB input devices, particularly my keyboard, stopped working.
On rebooting to an older kernel, the keyboard worked again. The reason for this, is that the base kernel package doesn’t include the usbhid module, which is require for USB input devices.
The solution, is to install the linux-image-extra package for your kernel. In my case it was:
sudo apt-get install linux-image-extra-3.16.0-28-generic
You can either do this via ssh, or boot to an older working kernel first.
Afterwards, you should be able to do
modprobe usbhid, or simply reboot, and your usb input devices should function correctly.