Sorry i made a mistake, your /etc/network/interface if using static ip should look like this:

auto lo eth0
iface lo inet loopback

iface eth0 inet static
address 192.168.x.x
netmask 255.255.255.0
network 192.168.x.0
broadcast 192.168.x.255
gateway 192.168.x.x (what was set on your router)