Please try the following:
1. If you're using a third-party theme or plug-in, it won't have an app icon display on the home screen, so check in the download files instead.
2. Restart your 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 vertical three dots and go to Show System. Then find the Nothing Launcher and clear the Storage & Cache.
4. Uninstall the corresponding App and re-install it.
5. Swipe up the home page to open the application centre and long-press on the app to drag it to the home screen.