Cant remember my wep passphrase

If it's an ISP supplied router, have a look at the base of the router, it may carry the default passphrase set by the ISP.

Alternatively you'll need the manual for the router and to read the section on addressing the setup pages within the router. Either connect your computer by ethernet cable or use a computer which has a working wireless connection.

Once in either read the hex version of the passphrase and apply that to your computer's wireless adapter interface -- or create a new passphrase and apply the hex translation to all computers using the network.

While you're at it you might see if the router and computers will support WPA encryption which is better than WEP.