If your device is not booting into Horizon OS after an update then you may try one of the three fixes from down below:
You may be running an old CFW version. Please ensure you are on the latest
CFW version. You may read this guide to learn how to update your CFW.
Your Switch not booting may be due a theme being broken on your updated firmware.
You can fix this by deleting the folder
If you have had installed previously any sysmodules, then please ensure you delete