If you delete a site there is no way to get it back unless you have a database backup and can restore the whole database. To prevent accidental deletion of sites we have hidden the delete button by default. You can enable it by setting this to true in Web.config or user.config:
<add key="AllowDeletingChildSites" value="true" />
Note that sites can only be deleted from the root site and the root site itself cannot be deleted. After you delete a site I recommend you disable the button again to avoid accidental deletion.