Your issue may have occurred for some reason where you are not able to see font-icons in font-icon manager, but you can simply reset your font-icon manager and have your icons back as you installed at first time.
Here is the video tutorial that will help you step by step that how you can reset your font-icon manager
Video Tutorail Link: https://www.youtube.com/watch?v=27Ii6FjHkzg