Recovering lost files from To Do app

I had more than 100 items including photos in my To Do app. Never had an issue accessing the app or reading my files until yesterday when I was asked for the first time to Sign In to my Microsoft account. When I did, I could not find any of my info in the To Do app. Has anyone been able to recover these files and how did you do it? My app is only on my iPhone, 8 plus.