Ethernet works half the time?

I've had at&t for a while. At least 5 years and never had a problem like this. My ethernet stopped working last wednesday. I am able to access some sites but not others. I cannot access online games but I can access online voice communication; my downloading still works fine. It says I must log in to this network before I can access the internet. Every time I do log in, (if it even loads) it tell me to pay to use the internet. I ALREADY PAYED. It keeps saying the same thing and it wont let me use my internet fully. The most frustrating part is it is only happening on my computer. My dad's computer (connected by ethernet to the same box), works just fine. All the devices work flawlessly. It is just this pc and I cannot figure out way. No things were updated on wednesday. I did not change anything. No settings configured, no new devices, no damage. It just stopped working and I am bewildered as to why. If anyone can help that would be great. I am willing to give more information if you need it.