Am I going crazy?

Hi all, my 87year old Mam is with Tesco Mobile pay as you go and she has always topped up via the app. But when I went to do it this morning for her the app no longer exists nor is it on the play store and when I go to Tesco's website and go that way I go to a dead end. Can anyone shed some light on this or what's the craic with it?

Thanks in advance.