Got an overpayment notice, what now? after 8 years as a janitorial supervisor

The agency says I reported earnings incorrectly, but I think I followed the instructions. Any experience at all would help. I did not notice any errors at the time. Who should I talk to first?

If you are unsure, free legal aid in your area may help for this kind of notice.

I asked for a waiver and a plan in the same call, and they gave me both forms. It was less scary than I expected.