Surprise Credit Card Debt

Today started with a knock on the door from the Sheriff's department.

After thinking that my old credit card debt had all been paid and/or written off, I was served with notice that Synchrony Bank is suing me for an thousands of dollars. I am a student who also works a full-time job, but each month I barely any money left over after paying my regular ongoing bills and the debt I *do* know about. Now I have 30 days to answer Synchrony's claim against me, which will involve paying as much as I can - which, at the moment, is zero.

Any and every little bit helps and would be greatly appreciated. I can't explain how I found myself in this mess, but I desperately need your help to get out.