First thing you must look to see if you have a Wireless or Wired set up. For Wireless users you should always first check the batteries. If the batteries are not the issue then the next thing we should do is try to unplug and re-insert the dongle to the USB port. For Wired users if the Keyboard/Mouse stops working your first and only thing you can try to do is unplug and re-insert the USB into the PC. For both Wired and Wireless you can always try a different port when reconnecting to get it to re-connect.