This is going to be a very short post but will be useful many developers.
It is registering one one startup script that will be fired when the page will be loaded.
Say you have used at several time and it was working but now it is failing. Similarly Page.ClientScript provide many other methods that will also not work.
So whenever we have an instance of ScriptManager on our page, we need to register the Script using ScriptManager. So you need to change the code like this
Now this code make the script start working. Similarly , we also need to change other Page.ClientScript methods to appropriate ScriptManager method.