Did you reboot the computer? Are you saying both keyboards don't work or the built-in one does not but the external one does? Does the mouse move OK?
If the external keyboard works, try re-connecting the keyboard, you'd need to take apart the case a bit for that of-course. If neither keyboard works, you probably have an issue with the motherboard ports and need it replaced, I'd suggest a BIOS flash but that would be a bit tricky with no keyboard.