The system will not allow the same login to be used in more than one instance. So, here are some things that typically have happened to cause this:
Perhaps you logged in, then closed out your browser without logging out. If you went to another computer and logged in, our system thinks you are already logged in.
If your time, date, or time zone settings are off, this can create a similar situation.
Sometimes, clearing the cache in your browser will correct the problem.