if root site collection is delete how to restore

hi Friends

          I have a problem regarding the site collection restore?I had deleted the the top level site collection through site settings in the site. I am not able to recover it. i had tried power shell command but that particular site is not visible in the power shell. when i tried to delete a site collection through central administration it is visible in power shell and it is restored.

Can any one help me...........  

sindu