For weeks now I have been using collapsible fieldsets to display a particular node type field without problems. Sometime during the last week it inexplicably stopped working. Collapsible fieldsets still work correctly for core stuff (e.g. module listing) and the legend element appears as it used to, only the fieldset is collapsed (as it should be) and can't be expanded (the legend isn't clickable). Inspecting the HTML I can see that the value that should be in the fieldset is indeed there. What's extra curious is that in the Views GUI the fieldsets work correctly but not on the frontend of the same view.
Edit: I have been using collapsible fieldsets successfully outside of views on manually created content. This is the first time I try to use them in a view and it was how I discovered they'd ceased to function, both on the view I'm working on and the PHP code I've written myself. As per Laxman13's post, collapse.js is not being loaded on my own pages for some reason.