Why won't my computer let me online?

Okay, my computer lets me do everything offline, but whenever I try to get online it says there's a connection error like I need an administrator or a new cord because this one broke, which is odd because I use my laptop, and it doesn't do it for my brothers computer, nor our iPods, which is what I'm on now. Could it be my computer? Or could a program not be letting me on? Both my brother and I have Acer Aspires and nothing is wrong with his, and mine just stopped working one day, and won't connect anywhere I bring it.