How do I force someone to pay be back money that I lent them?

I lent out 1200 dollars to my ex last month with the promise that she would pay me back on or before April the 14th with the money she was getting back from her tax return. We have all of this in writing with our signatures.. I had called and asked several times if she had gotten the return yet. As of a week ago she blocked her number so that I can no longer get through. Obviously her intentions is to not pay me back the money owed as its now April 16th and she's cut off all contact.