Without the actual recipe, I'm not going to be able to give specifics, but I can give some ideas on why the cookies might have come out hard.
Are they crumbly as well? The problem could be not enough butter. If they are flat the dough likely needs more flour (or you could have added to much butter). However, too much flour can also make them tough. Flour really needs to be measured precisely for the best end product.
If your cookies look good, you likely baked them too long or at too high of a temperature.
It's also possible that the dough was over mixed. Over mixed dough is not salvageable in my experience.
You could try storing them in an airtight container with a slice of bread and see if they soften up at all after a day.