I am just learning ionic and encountered this issue where my icons disappeared, but moved on since I am just learning. However, the issue was resolved when I followed the first 45 seconds of Leanne R's "How to use Ionicons with HTML and CSS | Icon Tutorial" video: https://www.youtube.com/watch?v=V03_ZwIzlvQ.

Solution was to add two statements from the ionicons usage page and icons started working again for me. Of course, the webpage says not to use these if you are using ionic framework which apparently is broken.