Hi DesoShade,
This could be a setting in your BIOS. When you next restart your PC, enter the BIOS by pressing 'Del' or 'F2' (or whatever key is required, usually displayed briefly on screen), and browse the options until you come across 'Legacy USB' (or similar). Set thisw to 'Enabled' for both keyboard and mouse. Exit your BIOS, saving settings, and restart your PC. Your keyboard should now be working. To carry out this procedure, you will need a PS-2 keyboard (or a USB to PS-2 adapter). If the motherboard is a non-legacy device, i.e. doesn't have PS-2 sockets, then try another keyboard. Before you do so, try resetting the BIOS to its defaults (see the manual for details on how to do this). If you have another computer with a USB keyboard that works, swap keyboards to see if it is a faulty keyboard. Carefully check the keyboard. Do any of the keys appear to be depressed? If so, that could be the cause of your problem. Gently prise the key(s) off and check the mechanism for dust and fluff that may impede its operation. However, keyboards are readily available and it might just be as easy to get a new one.
Dwarf