I get an IP address but can’t connect or

I have cable internet access, i am able to obtain an IP lease and renew from my ISP on DHCP, but when i try to log on i get an error msg saying that my modem is not connected check cables and connections.I have tried a new NIC, removed network support and reinstall, remove NIC drivers and reinstall, remove TCP/IP and reinstall, but problem still exists. Modem and cables are OK as im able to obtain an IP address.
Can anyone help?