There is a reset switch on the back on the modem that will revert the modem to it's default settings. Push a ball-point pen into the hole for 30 seconds to reset. You can then connect via wire to the modem and enter a new security key.
If you have a PC that does connect via wireless and just need to see the key - you can use this software to display the working key.
router key - go to the manufacturers web site and find the default ID and key. Then use the reset method from my prior posting. Then change the ID/password and wireless key - write it down and tape it to the bottom of the router.