To restore a site collection which has been deleted accidently , run the following PowerShell commands
1. Open the SharePoint 2013 Management Shell and type the following
a. Get-SPDeletedSite.
This will give you the list of site collections that were deleted in the last 24 hours. Make a note of the Site ID here.
b. Remove-SPDeletedSite –Identity <SiteId>.
This will remove the Site reference from the content database completely.
2.To restore the site collection.
a. Restore-SPSite –Identity <Site Collection Url> -Path <Backup Path>