My AIM won't show I am online?

I sign on with my screen name and everything, but it won't say I am online. I will see my buddy list, the people online, but they can';t see me online. I don't have the invisible feature on. I also can't send out IMs. Ive checked out everything nothign can be wrong. My friend signe don with my sn on another computer and it worked fine. Is it my computer or my firewall? What the problem is?