I tried to change the password for my wireless router and when I changed it, it kicked me off of the router gateway. after that I couldn't log into the gateway because the router doesn't work and I can't change the password help!
Most routers have a reset switch in the back. Hold it in for something like 4 seconds (go longer just in case), this will reset it back to factory defaults. You'll have to set it up all over again, but at least you'll be able to get back into it. Of course you'll have to dig out the manual if you don't remember the factory default login information, but no big deal.
This is a modem / router all-in-one? It may be more difficult on this equipment as there may be special settings on the modem side depending on the provider. I have one (I don't use the router section though) and the Telus setup is pretty much default so a reset wouldn't knock me off the internet. I use my own router because their modem / router doesn't support 802.11n or 5GHz.