By physical connectivity, I mean functioning Ethernet cables/connectors, and functioning NICs. You need bits to be able to flow along wires, before DHCP will work.
1) Are there status indicator LEDs for the NIC at either end of your Ethernet connection? If so, that will show whether the NICs are seeing electrical connections. 2) Check your Ethernet cabling -- bad cables/connectors are not uncommon. 3) Use an alternate port on your hub/switch/router if available, as NICs on such equipment can fail. 4) Swap NICs on your computer, if you can.