Hi,
I just finished working on a keyboard like this, this morning. It's all very straight-forward. First thing to try is just to pull the keys that don't work off, and put them back trying to make sure the spring goes up in the shaft made for it. When I put it back together, several keys didn't work, and you could actually tell that they weren't "springey". Just re-seating the keys fixed them perfectly. If you have to open it, watch the videos (there are others) - there's nothing to fear, it's all very easy and I suspect a cleaning will make it all work again. The keyboard is built like a tank.
Stefan