If you pull the power out of the back and leave it for 10 - 30 seconds that will cycle your router and then it will re-assign itself on your network and should be working......
If you mean reset it completely and start again then you can login to your router through a web page by going to the ip address it gives you normally underneath the router.....eg Billion normally has the IP of 192.168.1.254.
Underneath the router it would also have the default login details (probably admin/admin or admin/password) unless ofcourse you have changed it. If you dont know your password i guess your router would have a little reset button on the back that you could get to with a pin or something.
Once you logged into your router you can simply reset it to factory settings and change whatever you need, it would also contain details like login names for your ISP, but to get the password most likely you would need to ring up your provider and they will have it for you.
Was that what you were looking for?? Hope it helps