Well you're in luck. I happen to own one of those. ROM may be right, the inside might have electrical damage. However, these type of keyboards usually have a rubber membrane strip inside of them with conductive touch pads that make contact with the trace pads. Many times the conductor pad responsible for sounding that pad is dirty and dosen't make the connection anymore. If you use a "Q" tip with a little glass cleaner on it, you can usually restore the function.
The bad news is that because you seem to indicate that an entire bank of keys is out next to each other, the pad assembly or circut board assembly may be beyond repair. If you take the unit appart, work carefully, you will find many screws and possibly some clips that hold the circut board in place. I found a yamaha pss180 that was tossed for one key out. Inside I found a very small bug stuck in the pad that works that key. After cleaning the keyboard worked fine.
You will also find that the moist "Q" tip is usually enough to make the connection on the circut board and serves as a live tester for each key trigger. The tones were sounding as I cleaned the pads in the yamaha. I did this on purpose because I wasn't going to put everything back together only to find out one key dosen't work. Hope this helps you out. If push comes to shove, you can find a replacement on Ebay most likely.
If anyone is interested in seeing this classic board in action, check out Joe on YouTube playing the drums on it to a Boston song. Very cool. Here is the link and the search terms I used.http://www.youtube.com/watch?v=iao-Iq4Jtxs
Search terms for Google:
casio keyboard drummer jpizzle