Unfortunately, there is no user interface for restoring deleted site collections. Managing deleted site collections requires using PowerShell. SharePoint 2010 Service Pack 1 introduces 3 new PowerShell CmdLets for managing the Recycle Bin for site collections.
The easiest way to get a list of all the deleted site collections is to just call Get-SPDeleteSite without any parameters. In the following example, for brevity, I have formatted the output as a table and filtered the columns displayed. In the second sample, I have added a wildcard filter to the command that can be used to easily filter sites based on relative paths.
Once you have identified the site you want to restore, you call the Restore-SPDeletedSite. To restore one or more sites, it is easiest to just pipe the results from Get-SPDeletedSite. By default, the command will ask you to confirm the restore. To eliminate the confirmation, just add “Confirm:$true” to the end of the command.