Setting a CollapsiblePanelExtender state from CodeBehind
Usefully, the CollapsiblePanelExtender has a property called "Collapsed", this can be set in the aspx to whatever you want the default state to be. However, if you just set this property from CodeBehind, you will find that it ignores you and doesn't do anything. This is because you also need to set the ClientState property to be the same as the Collapsed property (Except a string, rather than a bool).
Like so:
CollapsiblePanel1.Collapsed = true;
CollapsiblePanel1.ClientState = "true";