Of PC, and operating system. USB or PS/2 mouse/keyboard? What do you mean, exactly, by "not working"?
More information will get more help.
My computer works, my monitor works, speakers, printer, but all of a sudden the keyboard and mouse are not working. I need help in knowing what to do. The hard drive is running and I can power on/off to verify that, the monitor just says check signal source cable? I have verified both mouse and keyboard are connected correctly to the pc. What else can I do?