The rules differ between the combined pill and the progestogen-only pill, and even between brands, so the leaflet that comes with your pack is the authoritative source. In general, take the missed pill as soon as you remember and continue the rest as normal, but the need for backup protection and emergency contraception depends on the type and timing.
The progestogen-only pill has a narrower window (often only a few hours late counts as missed for some types), while the combined pill usually allows more leeway. Missing pills around the start or end of a pack, or missing two or more, raises the risk more than a single mid-pack miss.
If you have had unprotected sex around the time of the missed pill, emergency contraception may be appropriate and is more effective the sooner it is taken. A pharmacist or family-planning service can give you tailored advice quickly. Use condoms as backup until you have confirmed you are protected again.