lake I need help with my internet connection on another computer?

I recently added another computer to my home. Last night I was able to establish an internet connection. Today, it says a network cable is unplugged. I've tried everything I can think of. All the cables are connected securely. I have a router with four ports. All the ports work and have been used. I even switched his ethernet cable with the port I am using. The ports still work. Any help would be great.there