being chased for CC debt

I have a few questions about CC debt.

I have 7 cards that are delinquent, some more than others. Total between 20K-23K. I was in agreements with all, making monthly payments. We slipped when things got tight again.

One card, a Citi, offered a settlement. It was ~35% of $6400, I don;t remember the numbers off hand. I made a payment of ~$150 to start it, then there were 3 payments to be made. I missed the payment date due to an expensive car repair, and didn't follow up.

Now I'm being contacted by a law firm acting as a collection agency. They've sent a letter, dated 9-17, and called a couple of times.

What are my options here? Can I contact the card issuer again, and get back on the settlement offer or has that boat sailed?

I'm getting a little nervous here. I really don't want to get into a court situation. So far, no talk of a suit.

I'm making payments on 3 of the other cards, so I'm OK there.

I live in Massachusetts, if that makes a difference.

Thank you...