I find the same issue as it is mentioned in this thread: Font awesome is not showing icon
Icon images are displaying like this sometime.
When I use this link:
<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.2.0/css/font-awesome.min.css" rel="stylesheet">
It works fine with the above link although I was using newer version of Font Awesome 4.7 which should work as it is the later version than in the above link. Also when I download the same version and upload to my server it didn't work but when I use the link then it works.
Also, this issue appears for the same page (most of time it works but sometime all icons are like this ), otherwise everything works fine and Icons are displaying properly
Is there any way to debug the issue? I replaced css files, uploaded newer version but couldn't find any solution yet that why sometime all icons disappears and replaced it with this image