Why is my keyboard not working?

The keyboard could have failed for several reasons but an easy way to test if it's the keyboard to try using an external usb keyboard.

From memory the keyboards are easy to replace if you have the know how but I would first attempt to re-seat the ribbon cable. (Disconnect and re-reconnect)

To find the correct replacement keyboard you can nomrally find it on the bottom of the defective part but can also google lenovo part finder and on the bottom of the laptop it will a type number (model code) which would be needed to find the correct part number.

Replacement 3rd party keyboards can be found easy enough once you have the part number.

Try to use an external Keyboard until you can find the solution to your problem. Once you login in your system try to update the keyboard driver some time an outdated driver can also cause these kind of issue as well.