Something that is "past its prime" is no longer as strong as it used to be.
You can use "past its prime" to talk about a country or city is "past its prime" when it is used to be economically strong but is now poor and run-down. You can also use it to talk about people who aren't as healthy and strong as they were in the past:
If something is still young, strong, and healthy, you say that it is "in its prime":
In his prime, he was one of the strongest players in the league.
Something that was in its prime a long time ago is now "long past its prime".