Today we got an issue, end user deleted sub site by mistake and they want us to restore the sub site immediately. Since this is a sub site I checked at Recycle bin for that navigate to Site settings
In Primary recycle bin we are not able to find that Sub site.
In the same URL we can able to see the secondary recycle bin as per the below screen shot
We are able to find that deleted sub site at secondary recycle bin as per the below URL
Select that particular sub site and click on restore.