If it's new, it's on the sticker or the manual. If it's forgotten, use the reset to reset the router and set it up again.
There is no known safe way to recover said key! Almost all such software is riddled with trojans.
I lost the WEP key for my Belkin N+ wireless router.
I bought a new wireless printer and cannot connect it through the router without the WEP key.
How can I get the WEP key #.