I'm not with Laminar Research, just a fellow simmer in the UK, but presumably you mean that you've deleted the X-Plane 11 default Boeing 737-800 folder in the C:\X-Plane 11\Aircraft\Laminar Research directory? If this is not what you mean, please advise?
If it is however, there are a couple of things you could try:-
1. Have a look in your Desktop Recycle Bin, as the whole Boeing 737-800 folder may still be there. If it is, right click the folder and select 'Restore' and the whole folder should be put back in the C:\X-Plane 11\Aircraft\Laminar Research folder. Alternatively click on 'cut', and then paste it back into the correct folder. WHATEVER YOU DO, DON'T SELECT 'DELETE' AT THIS STAGE,AS IT WILL PERMANANTLY DELETE THE FOLDER FROM YOUR PC ENTIRELY!
2. If you can't find the Boeing 737-800 folder in the Recycle Bin and you also purchased your copy of X-Plane from Laminar Research and not Steam, you will most likely have to run the X-Plane Installer.exe program again, which will then download and replace those folder/files that are either missing or corrupt.
3. If you purchased your X-Plane 11 from Steam, you may need to contact them directly for advice, unless Q&A guru Glen from down under can provide further guidance?
4. If you purchased the Boeing 737-800 aircraft from a third party provided, then simply download it again from their website and activate it with the licence key that you would have been supplied with initially.
Hope this helps.