Take her to small claims court to get your money back. Take this as a lesson learned and do not lend money to people. If you cannot afford to give them the money then you cannot afford to lend them the money.
If she agreed to pay interest then you have an enforceable contract. If she only agreed to pay back what you lent her, there was no valid contract and you will have a difficult time suing. To understand this, look up "consideration for a contract."