In SharePoint 2013 it is possible to restore a accidently deleted site collection. For more information, read this article: http://technet.microsoft.com/en-us/library/hh272537.aspx
You can use the Restore-SPDeletedSite cmdlet to restore a site collection.
However, if you removed the site collection using the Remove-SPSite cmdlet using PowerShell, the site collection will not be stored in a SPDeletedSite object.
This means you cannot restore a site collection that has been removed using PowerShell.