Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
What error do you get, or what happens?
Are you sure you are logged in to your Yahoo! account? that is of course necessary, to sending messages.
Also check your internet connection - e.g. by accessing a web page or two.
(I regret that I am not able to offer Live Phone Call service at this time; However, your request is still open to other Experts in this category, and one of them may pick up on it at any time.)
Try logging out of Yahoo, Signing Out of your Yahoo! account.
Then restart the computer (or handheld device).
Then when restarted, log back in. See if that frees up the account and the sending of messages.