Keys are not responding when I press down.

The keys you keyboard may not be working do something such as food or gunk stuck in the keys, you should check and see if this is the problem before trying anything else.

If this wasn't the problem you may have just worn out the keys, which will break after a while. The best way to repair this is to probably to find replacement keys and replacing them, however if it is the entire keyboard, you may have a more detailed problem.