My Twitter app I wrote last May now no longer works



I have a twitter app that used to work - I think about May 2016 I ran it last.

However now when I run it I get the error:
The remote server returned an error: (401) Unauthorized.

Also when I go to my Twitter developer page and click the “Test OAuth” button I get “Sorry, that page doesn’t exist!”

I haven’t changed anything - how can I find why it has stopped working?