One missed payment is usually a small problem treated as a large one. The consequences are mostly recoverable, and acting within a few days is what keeps them that way.
Nothing here is financial advice, and if payments are being missed regularly rather than occasionally, that is a different situation where free debt advice services are genuinely the right call and are worth contacting early rather than late.
The first 48 hours
First
Confirm it actually failed. A payment can show as pending, be retried automatically, or have gone out of a different account than you think.
If you can pay it
Pay it now. A payment a few days late is materially different from one a month late, and most reporting thresholds are measured in months rather than days.
Same sitting
Check whether anything else is due before your next payday, so you are not solving one and creating another on Friday.
If you cannot pay it
Call them. Providers have far more discretion before an account defaults than after, and almost none of that discretion is offered to people who did not get in touch.
Before you hang up
Write down who you spoke to and what was agreed. This matters if a different person tells you something different next week.
What actually happens, roughly
A few days late usually means a failed payment fee and nothing else. Around a month late is generally when it starts being reported. Several months is where genuine credit consequences and default processes begin.
The exact thresholds vary by country, provider and product type. The useful general point is that the gap between a few days and a month is enormous, and it is entirely within your control.
Calling them is the part people avoid
It is also the single most effective thing available, because providers have options before an account goes into arrears that they lose afterwards: payment holidays, revised dates, splitting a payment.
If that call is the thing you have been putting off for a week, that is an extremely normal response to it, and the preparation that makes it easier is in making a phone call you have been avoiding.
Why it usually happens
Very rarely because somebody decided not to pay. Almost always because the balance looked fine on the day, and a payment that had already been committed had not left the account yet.
That gap between what your balance says and what is genuinely yours is the actual cause, and it is explained in why your available balance is lying to you.