Not able to access accounts on my computer

Hello,
Since today, I've had a problem with my computer.. in the morning it just completely stopped and the entire screen went black. I had trouble trying to get it back working again. It's some sort of virus, but I don't know what type. Afterwards, when I tried accessing my email account, msn acc, facebook, etc, it wouldn't work. The page was not the "cannot find server" page either. It said that something was wrong with the server, and that if I keep going and trying to access my accounts, then someone from another party can read my personal information. I got these warnings both on Internet Explorer and on Mozilla Firefox. I don't know what to do, or how to get rid of this.