When another person expects something from you by a certain date, be it a meeting, or a debt to repay, or a work-related submission or something else, and you are unable to provide it by the agreed upon time, you owe it to that person to tell him you can't make it, explain why, and provide a plan for moving forward. It's a basic courtesy.
If you made an appointment to meet at a certain time and place, and it looks you're not going to be able to go, message the other party and tell them. If you think you're going to be late, tell them. If it's especially bad, ask if they would want to reschedule.
If you agreed to do some work by a certain time, but later you find you aren't going to make it on time, tell them as soon as possible, as soon as you find out you're not going to make it. Heck, maybe as soon as you think it's likely. It's better if you can propose some mitigation to minimize the impact of the delays.
If you owe someone money and you promise you can pay them back in monthly installments, but later you find you can't make the payments, tell them. Don't make the other person have to ask why payments are being missed. And if it's going to be a problem moving forward, say so, and maybe propose an alternative, or a new payment schedule that works for you. Don't just leave the other person wondering if they're ever going to see their money.
Obviously, it's better to just don't make promises you can't keep. But things happen, and often people will understand. And if they don't, then next time you should know better not to overestimate promises made to those people.
For me personally, this only applies if it's an actual person you've owed something to. If it's some corporation, it's probably not necessary - corporations like your electricity provider won't understand and will just cut you off instead of considering any alternatives. But for actual humans? It's a basic courtesy.