When you end your employment, you have 60 days to repay any outstanding 401(k) loan. If you don't repay the loan, it is deemed to be a distribution, and is subject to regular income tax plus an early withdrawal penalty if you are under age 55. At that point, since it is a distribution, it is no longer a loan. (Not only do you not have to pay it back, you can't pay it back. You can only deposit money in a 401(k) via payroll withdrawals. You can't legally put money in that account now for any reason, unless you convert it or transfer it or roll it over to a private IRA. And then any deposits are ordinary contributions subject to ordinary contribution rules and limits.)
