If you had not defaulted before your separation from service, this generally should have been handled as an offset distribution rather than a default. If that was done, the loan would have been satisfied by the offset distribution, the Form 1099-R would include code M in box 7 and you would have until the due date of your tax return to come up with the money and roll it over to a qualified retirement plan like the 401(k) or to an IRA to allow continued deferral of taxes on the amount rolled over.
If the loan was instead considered defaulted and you were issued a Form 1099-R that included code L in box 7, the loan remained outstanding after the default but became taxable as indicated on the Form 1099-R. In this case, subsequently paying off the loan is not a rollover and does not continue to defer the taxes on the defaulted amount but instead the repayment becomes after-tax basis in the 401(k). Eventual distributions from the 401(k) will then be proportionate parts pre-tax and after-tax so that you don't pay tax twice on the defaulted amount. You'll want to make sure that the 401(k) plan is tracking this after-tax basis properly so that future Forms 1099-R form the plan reflect the correct taxable amount.
