OK just to clarify: When you try to access the router via the webpage it does ask for a username and password yes? However you don't know what the username and password are..yes?
Assuming that is the case then as aaronic stated, tell us what the make and model the router is. You can find this on the underside of the router.
Not all routers do it, but some such as netgear display the default username and password on the underside of the router also. You may find you can login to your router using this username and password as you've never changed it. If it isn't displayed on the underside then it will be mentioned in the documentation that came with the router, assuming you still have it?)...
There is a way round it, in that you could reset the router but you'd probably lose all the settings that you'd put in for your other devices so I'd recommend that only as a last option.
Did you set this router up originally and is it your router?