Told I owe money back on my benefits - what worked for you?

I received a letter saying I was overpaid about $1200 on my claim. I am in St. Louis, in case local advice matters. 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 14 months ago, and it got easier once I kept a simple log of every call and form.