Will not open

I want to rate this app highly, but it wont let me. Twice I have downloaded the app to my iPhone 6 Plus, created a list of groceries that I want, then closed the app. Next time I go to the app, it will not open. I can swipe it, tap it, double tap it and yell at it, but nothing happens. Guess I will try it again, but it is very frustrating.

Prujoe about Shopping To-Do Pro (Grocery List)