What Windows version are you using on the PC?
Look under networking properties and for adaptor properties, there should be properties where you will see IP addressing that has 2 options, auto obtain and manual. It might be set on manual.
If your not getting any internet if directly to the modem or the router, then something on the PC might be causing the connection issues.
Disable any firewall or AV programs while testing.
Make sure the cables are good, sometimes the cables that you get with the modems and routers are not the best. I prefer CAT6 cabling. Keep us posted.