First check to make sure that no firewall or antivirus software is causing a connection problem that would disable your internet use. You will have to go through and disable each program that has those capabilities to test if the internet is being blocked by them. If this does not solve the problem then you will have to check to see if your addresses are being assigned properly from your router/modem.
Go to my network connections and go to your Local Area Connection. Then right click on your Local Area Connection and click Status.
To find out if you are connecting to the router/modem check the IP Address that you are receiving from your router/modem. If it is normal and looks something like 192.168.xxx.xxx then you are fine with this step.
If you have a router your default gateway will most likely be 192.168.1.1, these two things with getting a correct IP address from the router and having the correct default gateway automatically configured by your network setting will ensure connectivity to the router. We can still not have internet at this step even if we are connected to the router/modem.
To check if static addressing is being used go to Properties in your Local Area Connection Status and select the Internet Protocol (TCP/IP). Click Properties for the Internet Protocol (TCP/IP) and check to make sure that the button for Obtain an IP address automatically and Obtain DNS server address automatically is checked. In the Alternate Configuration tab Automatic private IP address should be checked. If you have static addressing set up in this picture than you will need to find out why it was put in place and figure out if that is where the problem can be occurring. It is rare that if a system works that one day the static addressing will go wrong, someone would have to go in and mess it up in order for it to stop working for this reason.
Once we verify that the computer is connected to the router/modem what we do next is login to the router/modem to make sure that all the settings are properly configured. We may need to login on a different computer where the internet and computer settings are properly configured. We can also reset the router/modem settings to default and reassign our security settings if the router/modem settings are suspected as the problem. Usually 192.168.1.1 is a routers address and a default password is set on the router when you first buy it. It is up to you to remember this password.
If we still have no internet on the computer use something like a USB WiFi Drive. Try to get wireless internet computer through the computer checking if the cabling or actual Ethernet card may be the problem.
If we still have no internet at this point what we do is change the port on the router if the computer is wired to a router, and if that doesn't work we change the Ethernet cable to a new one.
If the computer still has no internet you would have to deep cleaning of the computer by reinstalling all internet drivers and possibly Windows. May need to replace the router, cabling, modem, Ethernet Card and possibly more if hardware damage is suspected.