It is good practice to work our way down on the OSI network model, which means starting at the physical layer. You should check your cabling, and if necessary, you should replace suspect cabling with known good cabling. Having a working timedomain reflectometer, which can be used to locate discontinuities in a network cable, can be helpful in checking cabling.
The next step is to make sure that the ports are functioning. Even if they are physically functional, the layer 2 protocol (data link layer) may not be running on the ports you are using. On a Cisco switch, you can use the no shutdown command to restart a disabled port. You might also have a speed mismatch between the switch and the router; for example, ...
