Who is your ISP? If you PC is directly connected to your modem, the ISP is going to use the mac address of your computer to authenticate (I believe this is how it still works) if you then connect your router to the modem, it's not going to work (This literally just happened to me last week). Hook everything up the way it should be (Reset modem/router by reset buttons or unplugging, let sit for maybe 15-30 minutes) modem->router->pc used a cat5 if wireless isn't working. If you are getting an IP address ping the router (default gateway address). Also, be sure the modem is connected to the WAN port on the router.
Just a couple things to try, again I had the exact same issue. My problem, modem wasn't connected to WAN port on router (cheap $20 microcenter pos) (dark lit room ), one of the network ports on one of the PCs was bad, and the ISP had authenticated the wrong mac address (which was my laptop) So I was able to get on the internet via my laptop connected via cat5 but nothing else could.