Based on this little information, it sounds like there is not a route to the internet. The devices may have self-assigned IP addresses in the 169.254.x.x subnet. If this is the case, then the LAN side of the ISP router has to also be in this same subnet range and set as the default gateway on the internal devices. You need to take a look at the IP addresses and the gateway or router information to see where traffic should flow for various destinations.
Can you provide information about how many IP addresses your ISP provides? Your DHCP setup, your default gateway on your machines? Goes your cable modem also function as a router?
At first glance to me it sounds like you need to purchase a router for your network. Odds are a Linksys, DLink, etc will work just fine.