Rental Deposit money

<p> </p> <p>I was staying at , my contract ended and when i was getting ready to move they said i need to serve notice. How do i serve notice when the contract has ended... and therefore refuse to give me back my deposit. 10 days before the contract ended, she sent me an invoice with the rental amount and then 10 min after the invoice was sent she called to say rent has increased by 10% and sent another invoice with the 10% increase and she expected me to pay it that month. and I decided i dont want to renew my contract and she said i wont get my deposit back. Why must serve notice if i the contract is not renewed. </p> <p>Pay back the money!!!!!</p>