In their letter, is there a court and a case number listed? Not the attorney's file number, but a number assigned by the court. It might be called "C.A." or "Civil Action".
If there are no details like that, call the clerk's office of the court in the county where you live (might be the District Court), apologize that you don't have the case number, and ask them if there is a file for "secchange" as a defendant in a civil matter. If there is, ask them what the status is.
They can tell you whether or not you have been sued, whether or not they have a record of service, or whether the judgment has already been entered against you. Then we can figure out what you need to do.
I know this is really distressing, but there are lots of people here to help you!