Firstly you need to understand how to export data of a deleted site to .cmp file from the following article:
http://geekdeck.com/sharepoint-2010-recover-data-of-deleted-site-from-content-database/
Now once you have data in the .cmp file format you just need to import it to a new site so that your old content gets back to life.
Create a new site collection with blank template to restore the data of your deleted site collection
Open the SharePoint 2010 Management Shell by clicking Start > All Programs > Microsoft SharePoint 2010 Products > SharePoint 2010 Management Shell. Type in the following and press enter (change the paths with their actual values).
Import-SPWeb -identity http://yournewlysitecollection -path \\sharepoint\yourexportedbackupfile.cmp
Switch back to yout site and check the contents have been restored …
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