That is very unfortunate. I'm sorry to report that this issue cannot be resolved... please throw your Cybook away and buy a new one.
Just kidding of course
I've sent you the system files by mail. Please check your mail and just follow the instructions. That should resolve the problem. (If you mean by "unreadable memory" that you have wiped it. If it's really unreadable, i.e. defective, you'd need to have the Cybook replaced.)