This can be caused by any of the below reasons.

1. If you or someone else using your computer has recently cleared your Internet cache the favicons will be lost in many browsers because they're stored in this temporary cache.

2. Page has not been loaded recently and/or the browser does not look in the proper location for the favicon. Try refreshing the browser page (pressing the F5 key on the keyboard is a common shortcut key for doing this).

3. If you're the webmaster of a web page and are unable to get your favicon.ico to be displayed make sure the icon is stored in the root directory and if the page is stored in a folder or other directory try placing the favicon.ico file in that directory.

4. Finally, earlier browsers have had other various issues related to the favicon.ico file. Make sure you're running the latest available Internet browser version for your operating system.