If you are one day (or more) late in making your monthly student loan payment, your loan is considered delinquent. It will stay in that status until you make the full payment, change repayment plans, or request a deferment or forbearance.
If your loan is delinquent for more than 90 days, it will be reported to the credit agencies and will negatively affect your credit score.
If your loan stays in delinquency, it may go into default.