Your account is locked when someone enters an incorrect login multiple times (usually around 30). Yahoo! locks the account for "security reasons" and you are unable to access your chat names. Cool thing is, these are a piece of cake to unlock. All you need is a list of Yahoo! servers to log in with. Every time you login, you are using what's called a server to access Yahoo! You login with a cookie, which is sort of like an information packet or ID proving that you are who you claim to be, and the account is yours.
When people attempt to log your name in multiple times with a locking tool, all they are doing is logging in over and over with an incorrect password until your cookie expires, which means you will no longer be allowed to login. So, when you find yourself locked, what you need to do is refresh your cookie. But how can you do that when the Yahoo! server you were using has blocked you?
Simple, find another server. Yahoo! has servers all over the place. There are more than I'd care to count. I'll include a pretty decent sized list with this tutorial. All you have to do is paste one of these servers in your browser and log in on the page you see. Unless you have been locked on that server as well, you will receive a fresh cookie and will be able to log in to Yahoo! Messenger again.
Tons of people try to go through their e-mail and login there to unlock an account, but a smart locker will shut down your mail server first, then cut off your chat server. Most people don't know what to do if the e-mail is blocked, so they give up trying. That's why I don't even bother with the e-mail method. I simply pick another server, log in, and I'm good to go. There are so many servers out there that it is impossible to lock them all. Even if they could, the lock is only for about 12 hours, and then you're back in business.
Some Yahoo Servers :http://cn.mail.yahoo.com/?id=77070