When a Facebook ad account reaches one of its limits, it can no longer publish new campaigns. If one of your campaigns fails to publish for this reason, you'll receive an email like this:
Your account has reached the maximum number of ads allowed on one account (5000). Please delete some of your old ads to create new ones.
Ad accounts have a limit of 5000 ads, 5000 adsets, and 5000 campaigns. When a limit is reached, you need to delete some old campaigns to free up space and be able to use the ad account again.
Once you've deleted and freed up space in your ad account, try publishing your failed campaign again. Click the link you received via email, or edit the campaign and navigate to step 5 for publication.