OK. First of all, this could just be due to the Yahoo servers being down or busy. They have been making a lot of changes to them in the last few weeks.
However, YAHOO Messenger has been having many invalid login password problems lately. If you are unable to log in to Messenger but are able to log into your account to access information, mail, and My YAHOO then please visit the following web page and fill out the form to notify YAHOO so that they can fix your account as soon as possible. Be sure to mention that you can access all other areas of your account with the exception to Messenger.
http://help.yahoo.com/help/us/mesg/mesg-25.html
It may also help for you to clear your temporary cache, although from the sounds of it, the Yahoo servers are just temporarily down or busy.