https yellow warning triangle

Overnight, after 1900+ posts, my site suddenly shows a yellow warning triangle over the https lock. It shows up on all of the pages. I’m not tech savvy at all, but have tried to find an improper link and can’t. The site is https://thepoetrydepartment.wordpress.com/ and I’m using Firefox.
(Apologies…I spent an hour looking in all the likely places for the version of WordPress, but can’t find it anywhere.)