Can't put in wireless key

Which key -- the router's admin login passphrase or the wireless security passphrase?

Have a look at the router's manual and figure out how to address the router's user interface.

If the admin login password has been changed from the factory default you need to reset the router.

If it's merely the wirless security passphrase that you've lost -- hook up to the router via an ethernet cable instead of wireless and change the wireless passphrase.