Some HP and Dell Windows laptops are plagued by a problem in which the keyboard and possibly trackpad stops working for no evident reason. The keyboard seems to work OK on BIOS screens and other pre-boot screens, but after Windows boots the keyboard is non-responsive. Attaching an external USB laptop will allow you to use the laptop, but it is a clumsy way to address the issue.
Often there is a quick workaround for this that you can try in just a few moments. First, shut down the laptop completely. Then, unplug its power cord. Next, flip it over and remove the battery. Flip the laptop back over, and locate the power-on button. Hold this button down for a good 30 seconds. Let go of this button, reinsert the battery and power cord, and try rebooting the laptop. If luck is on your side, this may have temporarily cleared up the keyboard issue.
If this fix seemed to work temporarily, your laptop manufacturer may have a new version of the BIOS available for download that fixes the issue permanently. After completely backing up your laptop, visit the manufacturer’s support website and see if such new BIOS version is available that lists this symptom and its fix. If so, you may wish to try the BIOS update. Updating the BIOS carries some risk, and should only be done if you are confident that you have a complete backup of the laptop, and are willing to assume the risk that the BIOS update will fail and render your laptop unbootable.