unplug it, then plug it back in. if it's still not working check device manager for HID (human interface devices) see if it's showing up there and if there are any warning labels next to it. or if there is a device listed that is unusable due to no driver or such.
if it's not listed and there's no driver warning labels it's dead. if there are warning labels it could be a bad driver loaded and it needs to be uninstalled or reinstalled with the proper driver.
if you spilled a liquid of any kind on it and it quit working, you shorted out the circuitry and it's shot, go buy a new keyboard.