Is the router new or used? If used, reset it back to factory defaults. You do this via a recessed button normally located beside power plug on the router. I use a pen to push in and you'll feel a button. Most routers you do this with the power plugged in, hold it for 5 seconds or so, and all the lights will come on. You release it and it's reset.
Plug your PC's ethernet cable into a LAN port on the back of the router. On the PC, open a CMD prompt and type ipconfig /release press enter then type ipconfig /renew press enter. This renews the IP address, when it finishes it will give you the new IP, sub net mask, and default gateway. The default gateway is the address you enter into a browser to gain access to the router along with the default UN/PW.
If the router is new out of the box, connect power and the ethernet cord to a LAN port and your PC. Open a CMD prompt and type the ipconfig /release and /renew commands like above, noting the info displayed.