Not sure what brand you have, hope this helps!
Lynksis:
http://technicallyeasy.net/2008/01/how-to-reset-linksys-wrt54g-wireless/
Belkin:
http://www.dslreports.com/faq/13631
Netgear:
http://kb.netgear.com/app/answers/detail/a_id/1148/~/default-password-for-netgear-devices