I'd take the tops of the keys off (rip them off as quickly as possible, because pulling them slowly can damage the plastic components underneath). Once the tops are off, you'll see a bouncy rubber piece. If you press on this firmly with your finger and the letters do appear, then I would say the issue is with the plastics under the key probably needing to be replaced, and for that you can find people on eBay who offer individual keys and the plastics underneath them for $5 or so.
If pressing the rubber trigger does not work, unfortunately there may be damage to the circuitry of the keyboard, and you may have to replace it. This is most common with liquid damage, I'd say.
You can put the keys back together by lining things up, and pressing down until you feel and hear a "click". Don't press too firmly, or you may cause damage. Good luck!