Whenever I try to sign on messenger, a pop-up says: "could not sign on, already signed into other device or computer"

I don't think I am signed in anywhere.
I got this problem right after I changed my password to get onto my computer. That p/w is different than the one I am using with Yahoo mail, which works fine.
In fact, the messenger and mail p/w are the same.
Is it saying I can't use messenger with my mail being open?
I still can't get in when the only thing open IS messenger. THEN, it sends me back to the page where you get a new password or user ID.
Thank you for your help!
Its driving me nuts.