The slot came and went and nothing appeared. Start with the bell in your primary nav. When a Series skips a post it sends you a notification with the reason, which usually names the problem outright. A Series builds each post up to two hours ahead and uploads it at the slot time, so something a few minutes late is often still on its way.
If the channel is the problem, open Settings → Channels. A channel marked Reconnect needed can't upload anything until you press Reconnect. A channel that's over your plan's channel limit is skipped the same way.
The post might also be waiting on you. On the Series page, Before publish reads either Auto-publish or Manual approval. On Manual approval the post renders on schedule and then sits in your Library, unpublished, until you publish it yourself. Whatever that setting says, the first post of a Series is always held for review. It waits on the Series page until you press Approve & start schedule.
If there wasn't enough usage when the render started, the slot is skipped, and the notification says there wasn't enough usage left for the post. That includes the case where you have usage packs but Extra usage is off. Buy a usage pack, or switch Extra usage on at Settings → Plan & billing, before the posting time and it still goes out. After that the Series moves to its next posting day.
If it all looks right, give it a few minutes. A publish that hit a temporary YouTube error is tried again on its own, and Chillframe looks every five minutes for publishes that are due or stuck. Still nothing? Reply to this or email [email protected] with the Series name and the slot time, and a human will read it.