This seems to be a fairly common problem with Vista. It might help to give your Laptop a fixed IP address, because the most likely reason is that it has lost its IP address, the router is not aware of this and it isn't automatically renewed. Replugging the cable will prompt the router's DHCP server to renew the IP address.
To allocate a fixed IP address, open the Network Connections folder ("Start > Run", type NCPA.CPL and click OK), highlight the Local Area Connection, right click it and select Properties. Highlight the entry for "Internet Protocol Version 4 ((TCP/IPv4)" and click the Properties button. Click the option "Use the following IP address" and type in:
IP address - 192.168.x.7 (where "x" matches the third octet [number] of your *ROUTER'S* IP address)
Subnet Mask - 255.255.255.0
Default Gateway - 192.168.x.1 (your *ROUTER'S* IP address)
DNS Server - 192.168.x.1 (your *ROUTER'S* IP address)
NOTE: To revert to automatic IP addressing, set *BOTH* IP and DNS sections back to getting their addresses automatically.