When a Series can't render or post, it skips that slot and moves to the next one. Skipped slots are never posted late, so the Series takes the next slot in its posting window rather than doubling up. You get a notification in the bell in your primary nav, titled "[Series name] skipped today's post", with the reason underneath and a link back to the Series. It reaches you as long as in-app Publishing notifications are on at Settings → Notifications.
The usual reason is usage. The note says there wasn't enough usage left for the post. If you have usage packs but Extra usage is switched off, they aren't touched, and the note tells you to turn it on. A post starts rendering up to two hours before its posting time, so buying a usage pack, or switching Extra usage on at Settings → Plan & billing, before then still gets it out. After that the Series moves to its next posting day, and carries on by itself once you have enough usage for a post.
Next most common is the channel, and the note tells you which it is: YouTube needs reconnecting, no channel is connected, this Series has no channel chosen yet, or a downgrade left you over your plan's channel limit. Sort it at Settings → Channels, where a channel that lost access shows Reconnect needed and a Reconnect button.
A slot can also pass because too many renders were already going. Starter runs one at a time and the bigger plans run more, so a slot that comes up while another render is under way gets skipped. A Series set to Plus, or to 1440p or 4K finishing, is skipped on Starter too, because those need Creator or higher. Posting also stops while a payment needs fixing, while your plan is paused, or when there's no active plan, because Series need a plan even if you have usage packs. Sort that at Settings → Plan & billing and the next slot goes ahead.
One case isn't really a skip. If your Series is set to manual approval, the post rendered fine and is waiting for you: the first one on the series page with Approve & start schedule, and every post after that in your Library, where you publish it. For anything else, fix the cause and the Series carries on by itself. If the note in the notification doesn't match any of these, email [email protected] and a human will read it.