In most places your landlord is required to send a statement indicating what expenses were paid with your security deposit. This usually has to be done within a certain number of days after you move out. If you never got a statement, you should first read your landlord tenant laws and then if your state has those laws file against your landlord in small claims court. Provide proof to the court that you paid the security deposit and also a copy of your lease agreement. Show up and you will prevail.