Told I owe money back on my benefits after 8 years as an event coordinator

I received a letter saying I was overpaid about $1050 on my claim. Any experience at all would help. The letter mentions an appeal window and a repayment plan. Who should I talk to first?

Thanks for writing this out. I went through something close to it last year, and it got easier once I kept a simple log of every call and form.