Overpayment letter from the unemployment office after 15 years as a line cook

After 20 weeks of benefits I got a notice saying I must repay some of it. I have read a lot of threads here and could not find my exact situation. I did not notice any errors at the time. Who should I talk to first?

Thanks for writing this out. I went through something close to it 12 months ago, and it got easier once I kept a simple log of every call and form.