Why is one strip of my keyboard not working?

I agree with Klaus, damage from liquid, or just plain broke. Could be the keyboard itself, or the ribbon cable that connects the keyboard to the motherboard (less likely). Most laptops come apart pretty easily with basic tools and the keyboard can be changed. Check ebay for a new keyboard for your machine. Youtube might have a video on how your specific laptop comes apart. Some older laptops have keyboards that can be removed without pulling the whole laptop apart; there are little clips at the top of the keyboard that can be released with a small flat blade screwdriver, and in some cases a few screws that go through the bottom into the keyboard that hold it in place.

@bethlehem besides all those great answer already given, here are my 2 cents. Get the service manual for the Flex 6-14 from here That will help you to work on it.

The issue here is that Lenovo, as well as all the other makers, have opted to solidly connect the keyboard to the upper case (usually via rivets, either metal or plastic), with the idea that you need to replace the whole top cover/palmrest for this. If you find just a replacement keyboard, you could try and remove those rivets and replace the keyboard only. You will then need to secure the keyboard with a boat-load of small screws. Otherwise you have to replace the complete top case. No easy task but possible

Well Dust, dirt, hair, and other debris can fall into the keyboard over time and obstruct a key's movement or interfere with its circuitry. Try removing the key that isn't working, and clean the area under and around it. And also check for outdated keyboard driver can also cause the this issue as well so I will suggest you to check your keyboard driver if it is outdated then updated the driver.