Twice now when I am on email, doing 'Chat', I have been disconnected?

It tells me that I have been disconnected because I signed on to 'Yahoo Messenger' from another device. I can do 'Email', and dabble in 'Chat', but have never done 'Messenger'. I would not know how. It freezes my computer. What is happening to the computer? I do not have a wireless. I do not share the computer. Can I find out who is causing this problem?