Please follow the below steps:
1. If using a third party theme or plug-in, it won't have an app icon display on the home screen. Those will be available in the download files.
2. Try restarting the phone. In many cases, the home screen will refresh, and the icon(s) will return.
3. Go to Settings>Apps>See all Apps. In the upper right corner, tap the [three-dotted line] > Show System, then find the Nothing Launcher and clear the Storage & Cache.
4. Uninstall the corresponding App and re-install it.