If your keypad does not respond after power on, and Home screen looks normal, and NO keys respond, then it is likely not a key problem. It may be a software problem.

You can try a Hard Reset of your phone. See my previous C500 problem responses as to how to Reset your phone.

If you are half-way handy, you might try looking for an obvious hardware problem by disassembling phone to point of keypad cable-to-printed circuit board interface. Clean interface and unplug/replug cable. If this works, it will save you from losing your personal data and app files due to a Hard Reset of the software.