It's happened to me ever since I got my first router. The more stupid passwords I have, the more I forget. Maybe it's one of Murphy's laws?
Anyway, the easy answer is reset the router. Find the reset button, press it for 30 seconds, and it's all back to factory defaults.
For Linksys/Cisco routers, head to their website to grab their manuals (usually PDF format) and firmware ("ROM") updates.
I've also had Dlink network hardware in the past, so here's their support/downloads link.
