How do you stop a session?

I cannot log in to my account. My students cannot log in to their accounts. This happened in the past and I was told I had too many sessions open. I do not know how to close the sessions. Is their a button I can click on that can tell me if I have more than one session open?

I have the paid account. I thought that was the problem before, but it is happening again. Please let me know what to do. 

Thank you,