This happens occasionally to me. What you need to do is reboot your modem and when it comes up reboot your router. This should solve your issue. If it does not makes sure the router has a IP address and you have not created any firewall rules to block all traffic on port 80. If the router is good make sure your workstation is getting an IP address as well. If you are not make sure DHCP is turned on (network properties, tcp-ip, IP V4). If DHCP is off that IP address will take precedence even if it is incorrect.
If you still have issues let me know the results from the above tests.