A:Answer Hello ChrisG! Sorry to hear you're having issues with K540 of your MK540 Combo. Couple things to check, when flipping the On/Off switch at the top right of your keyboard, do you see a light come up right next to it? If so, that mean the battery if working fine, so maybe it's just a pairing issue. What I would suggest you to do is try downloading the Logitech Unifying Software https://support.logi.com/hc/en-us/articles/360025297913-Logitech-Unifying-Software, install it on your computer, and follow the steps to make sure the K540 is properly paired with the Unifying Receiver. If you see that it is paired with the Receiver and still isn't working, then I'd suggest getting your Combo exchanged if you're still under the store warranty, or file a warranty claim with Logitech directly if you're still within your 1 year of warranty. - Mathieu from Logitech