Does it show up in the sound options in control panel? If it isn't it may not be plugged in ( The wire that goes from the keyboard to the mic on the screen area (If thats the way your laptop is setup that way) could be severed)
Or the drivers
disappeared so its not being recognized. You can usually find drivers at your manufacturers website.
Sorry if this doesnt help but this is all I could think of, hope you other guys can help more