Granulated, hardened homemade caramel sauce
Most likely, it crystallized because something with a sugar crystal on it contacted the caramel while it was cooling.  This is why you need to change spoons a couple times while making the caramel, and take other steps.

If your caramel sauce is just sugar and water, you can remelt it, adding a water to it.  However, if you added cream to it, it's a bit tricky, because those dairy products will burn at caramelization temperature.  You may be better starting over.

However, there's another possibility: if you overcooked your caramel, it will have turned into toffee, which is also grainy and hard.  In this case, enjoy your toffee, because there's no turning it back into caramel.  You'll know that this was your problem because it's an intense dark brown.