How to have Safari play again Flash videos?
One solution :

Kill the process Safari Web Content. Warning : In all pages, you will lose all the data you may have entered.

Explanation :

The process Safari Web Content has just been killed without human intervention. And Safari reloads all the pages. This is a bug in Safari. It happens from time to time, when many pages are loaded, it is a kind of process apoptosis. Now, I have the Flash Player plug-in process running, and Safari loads the Flash videos. :-) 

Confirmation : 

The bug has occurred again. This time, I have killed the process Safari Web Content myself. This solution works.

[Edited : Confirmation of the solution]