Do you have any other computers on the network, wired or wireless? Do they work?
Can you access anything else over the network? A good test would be to go to your browser and try to access your router. Just type in the IP address of your router. For most, it's 192.168.1.1, but some brands are 192.168.11.1, 192.168.2.1, etc... Your router's manual should say what it is.
Or, if you go to Start > Run > CMD > ipconfig
Make sure your IP address is within the range of your gateway. IE if the gateway is 192.168.1.1, your IP address should be 192.168.1.x, x being between 2-254. If this is correct, the gateway is your router's IP address. If you hadn't already, try typing it into your browser and seeing if you can access it.