If you followed Plailect's guide to the very end, then this is a very easy to fix.
Hold Start at boot time. Depending on when you followed the guide, will depend on whether you get Decrypt9 or Hourglass9.
You can find Universal Inject Generator in Plailect's guide.
Link:
https://github.com/Plailect/Guide/wiki/Part-4-(2.1.0-Downgrade)
If it's Hourglass9, then follow these instructions.
- Open Hourglass9, then go to "SysNAND Backup/Restore", then select the "Health & Safety Dump" option to dump Health & Safety to hs.app (you can use Up and Down / Left and Right to change the name)
- Press Select to eject your SD card, then put it in your computer
- Extract Universal Inject Generator, then copy both hs.app from your SD card and FBI.cia from the FBI zip to the input folder
- Double click go.bat (or execute go.sh with Terminal on Linux / Mac)
- Copy FBI_inject_with_banner.app to the root of your SD card and reinsert your SD card into your 3DS
- Press B on Hourglass9, then go to "SysNAND Backup/Restore" and select the "Health & Safety Inject" option
- Press down once to select FBI_inject_with_banner.app, then press A and confirm to inject
- Go to the main menu, then press Start to reboot
If it's Decrypt9, then follow these instructions.
- Open Decrypt9, then go to "SysNAND Options", then go to "Miscellaneous...", then select the "Health & Safety Dump" option to dump Health & Safety to hs.app (you can use Up and Down / Left and Right to change the name)
- Press Select to eject your SD card, then put it in your computer
- Extract Universal Inject Generator, then copy both hs.app from your SD card and FBI.cia from the FBI zip to the input folder
- Double click go.bat (or execute go.shwith Terminal on Linux / Mac)
- Copy FBI_inject_with_banner.app to the root of your SD card and reinsert your SD card into your 3DS
- Press B on Decrypt9, then go to "SysNAND Options", then go to "Miscellaneous..." and select the "Health & Safety Inject" option
- Press down once to select FBI_inject_with_banner.app, then press A and confirm to inject
- Go to the main menu, then press Start to reboot
Edit: Pretend it says EmuNAND instead of SysNAND if you are on using EmuNAND with your A9LH setup.