Money owed to me

Nashua mobile closed down and I am battling now for months to get my money back from them.I even had to send them proof that I did pay them..I have been a customer with them for years so why must I sent them proof now that they owe me money but I send the proof.I have been communicating with a Shahida and aftr lots of emails she has stopped replying.I even called them and lately she is out of office when I call.