How do I fix this then???
I'm not really sure on how you'd go about finding out your WEP password.
Did it not tell you when you changed your Security Type to WEP?
I'm not particularly savvy with routers and whatnot, and never had much luck with WEP and DS's. (Gave up until I got my 3DS and it can use WEP and WPA).
What router do you have? That might be able to help someone else with working it out.