Do you KNow your admin id and password for the modem/router? Not sure what it is for DSL but for FIOS routers its id "admin" password either the value listed on the router for password, or the serial number if no password is listed. Once logged in you should be able to view the WEP key and if you wish least change it. Note if your modem/router supports it and all your devices support it, seriously consider changing to WPA2 or at least WPA for wireless security rather than WEP.
All modem/routers I know of also have a reset button that you can hit to have the basic settings, including passwords) go back to their defaults.
