If a repayment might slip
Talking to the lender before the date usually leaves more options open than after it.
If you can see a missed payment coming, contact the lender before the due date. Some lenders can move a date or accept a smaller payment for a while. They can't offer anything if they don't know.
A payment that's already late can bring a fee and may show on your credit history. Keep a note of who you spoke to and what you agreed. If several debts are pressing at once, a non-profit credit counselor may be worth talking to.