If you’ve checked the batteries and they’re all good then you may simply need to re-pair the keyboard with the receiver. Most wireless keyboards will have a connect button on the bottom and the receiver will have a similar button too. Usually you press the button on the receiver first and then the one on the keyboard second to pair the devices.
If this doesn’t work remove the USB receiver and reboot the computer with it unplugged, once the system has restarted try reconnecting the device, if Windows makes the noise to acknowledge that you have connected a USB device try repairing the receiver with the keyboard to see if this resolves the issue. If none of the above has worked the keyboard may be faulty.