If you completely stop making payments on your federal student loans, they will become delinquent the first day after a missed payment. Federal repayment plans usually have a payment window of 15 days, so this is when the amount will officially show as past due. Even if you miss just one monthly payment and then start making payments again, your loan account will remain delinquent until you repay the past due amount, or make other arrangements with your servicer (like requesting a forbearance or different repayment plan). If your past due balance has still not been paid after 270 days, your loan(s) will go into default.
Student loan defaults are reported to national consumer reporting agencies, which will negatively impact your credit rating and future borrowing ability. In addition, you may be subject to legal action like wage garnishment and/or the withholding of your tax refunds, which is known as Treasury Offset. You will also be unable to apply for additional student aid should you return to school if your loans are in default.
If you believe your loan has been placed in default by mistake, confirm with your loan servicer that your loan payments, deferments, and forbearances have all been processed and documented correctly.
Please read this article if you are worried that you're nearing default on your federal student loans.