The approval notice is actually the wrapper of the physical EAD

The approval notice is actually the wrapper of the physical EAD card
http://blog.wenxuecity.com/blogview.php?date=200710&postID=2480#I765EADAN

Unless it is returned to USCIS, the EAD will not be replaced. You will need to apply again on your own.
The authorization to work granted to you by USCIS, is not terminated by missing of EAD.