If you are a W2 employee, it is probable that your employer deferred and that your employer will pay the funds back by withholding from your wages and sending them to the IRS.  You can contact your employer for details.

If you are self-employed, check your return to see if you checked the option to have the payment deferred.  Then pay the deferred funds, if any, by using IRS EFTPS.

You choose to defer part or all of the SS taxes ... the program doesn't make the election automatically ... do you remember these screens ?