The sheriff is enforcing the judgment. It is difficult to tell you what to do because you have not stated what is in the letter. It could be a wage garnishment or it could be a letter stating that you must provide information about bank accounts and assets that the Sheriff seeks to sieze.
If the judgment was by default and you have a defense you may be able to attack the judgment and have it vacated, which will undermine the authority of the sheriff to collect.
When you find out that a judgment has been entered against you, normally by discovering that your wages are being garnished or the bank account has been frozen, you still have a couple of options.
In most States the Courts allow you to reopen and challenge a “default judgment”. Most States will allow a year to accomplish this. If you did not get the paperwork for the lawsuit, or you got the paperwork but never did anything, then the judgment was “entered” due to your default to file an Answer.
You have to bring a motion in the Court where the judgment is filed. That motion normally has to include a defense to the charges and a copy of your proposed Answer.
You should definitely get a local lawyer to help you with this, or seek the assistance of a Bankruptcy attorney to get rid of the liability. If you can’t afford a lawyer and you are being sued check out my website for a free form Answer.
Disclaimer: This answer does not constitute legal advice. I am admitted in the States of New York, New Jersey and Massachusetts only and make no attempt to opine on matters of law that are not relevant to those three States. This advice is based on general principles of law that may or may not relate to your specific situation. Facts and laws change and these possible changes will affect the advice provided here. Consult an attorney in your locale before you act on any of this advice. You should not rely on this advice alone and nothing in these communications creates an attorney client relationship. The opinions expressed herein are those of the author only and the fact that he has worked as an Assistant District Attorney; State Supreme Court Clerk; Special Assistant United States Attorney (Hawaii); Assistant Cornell University Counsel or Judge Advocate, United States Marine Corps should not be relied upon to assume that these statements reflect the policy of these organizations.