I agree, we need more information. What did you do prior to these problems occurring? What information can you give us when 'some' of the keys worked?
If your keyboard uses drivers (and I know few that do), try uninstalling them and reinstalling them, or even rolling back your system. Also make sure there's no physical damage to your keyboard. You may have damaged it's cable or PS/2 USB connector.
Although unlikely, check the port that you connect it into and make sure it's in working condition (plug other devices in etc.) and make sure it's free of dust.
Hope your able to fix your problem.