Actually I did work and I filed my taxes, but I didn't end up having to pay anything and instead I got a refund.
Are they saying basically that I didn't file my taxes or no?
So what kind of income did you have? Who did you work for?
If you have earned income--the only kind that can create a refund for EIC and child tax credit--you had to have fica/mc taxes paid. If it wasn't paid by your employer, then it's your responsibility to pay it.
Social Security tax is different from income tax. Did you work as an "independent contractor" and get a 1099 (instead of a W-2) from your "employer"? That could be the reason why. The other possibility is that you were an employee and the employer failed to pay Social Security payroll taxes.
if the letter said that Social Security taxes were not withheld it meant that the employer failed to withhold and that is a big problem for him
it is possible SS Admin will require you to pay the portion that should have been withheld from you
who sent the letter to you, Social Security or Internal Revenue?