Well this is a big question as there are many things it could be. First try to make sure your web browser is configured properly. So in Internet Explorer, click 'Tools' --> 'Internet Options'.
Then a new window will appear. In that window you will see tabs (or known as bars or odd type of buttons) lined up at the top of the window. Check if the 'Connections' tab is configured. The 'Connections' tab should only have used settings if you use a proxy server or if you use dial-up.
Also check if there is connectivity in the network connections under the control panel. To do this click 'Start' --> 'Control Panel' and on the side click switch to classical view if it is not already. Then Find the icon/pic with the words 'Network Connections' below it. Double click that icon/pic with those words below it and check if your network connection has a red cross on it. If there is a red cross on the connection to the network and your internet is turned on then you will need to configure it to read the network. To do this, single click on the connection that has the red cross (if the network is turned on) then you will see the sidebar to the left change. On that sidebar single click on the text that says 'Change settings of this connection'. A new window will appear and that is where you need to configure the settings. Depending on your internet service provider, you may be provided with what some settings should be set to. Especially if you are using a proxy server. I will be brief on this one since it is such a general question. In the general tab (at the top) you should only need to configure the option that says 'connect using' if it has below it the wrong hardware device. You shouldn't have the worry about the 'advance' tab unless you want to change the settings in relation to the windows firewall. Also you may find other tabs which can vary from name to name as it relates to how the networking hardware is used. So make sure that the settings there are correct especially if you are using a wireless network.
Also check if the computer can connect to the modem or the local area network. If it can't connect to the modem login page (in the web browser) then check that all cords are plugged in properly and that any software provided by your Internet Service Provider is installed correctly and configured correctly. Let me know if you need further assistance as it is a very general question and that you have provided so little information about the problem.
